Glossary of epidemiology

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A

Sample epidemiology chart - small intestinal cancers by incidence and prognosis

• age-adjusted mortality rate. 

a mortality rate statistically modified to eliminate the effect of different age distributions in the different populations.

• agent. 

a factor, such as a microorganism, chemical substance, or form of radiation, whose presence, excessive presence, or (in deficiency diseases) relative absence is essential for the occurrence of a disease.

• age-specific mortality rate. 

a mortality rate limited to a particular age group. the numerator is the number of deaths in that age group; the denominator is the number of persons in that age group in the population.

• analytic epidemiology. 

the aspect of epidemiology concerned with the search for health-related causes and effects. uses comparison groups, which provide baseline data, to quantify the association between exposures and outcomes, and test hypotheses about causal relationships.

• analytic study. 

a comparative study intended to identify and quantify associations, test hypotheses, and identify causes. two common types are cohort study and case-control study.

• applied epidemiology. 

the application or practice of epidemiology to address public health issues.

• association. 

statistical relationship between two or more events, characteristics, or other variables.

• attack rate. 

a variant of an incident rate, applied to a narrowly defined population observed for a limited period of time, such as during an epidemic.

• attributable proportion. 

a measure of the public health impact of a causative factor; proportion of a disease in a group that is exposed to a particular factor which can be attributed to their exposure to that factor.

B

• bar chart. 

a visual display of the size of the different categories of a variable. each category or value of the variable is represented by a bar.

• bias. 

deviation of results or inferences from the truth, or processes leading to such systematic deviation. any trend in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that are systematically different from the truth.

• biologic transmission. 

the indirect vector-borne transmission of an infectious agent in which the agent undergoes biologic changes within the vector before being transmitted to a new host.

• box plot. 

a visual display that summarizes data using a “box and whiskers” format to show the minimum and maximum values (ends of the whiskers), interquartile range (length of the box), and median (line through the box).

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C

• carrier. 

a person or animal without apparent disease who harbors a specific infectious agent and is capable of transmitting the agent to others. the carrier state may occur in an individual with an infection that is inapparent throughout its course (known as asymptomatic carrier), or during the incubation period, convalescence, and postconvalescence of an individual with a clinically recognizable disease. the carrier state may be of short or long duration (transient carrier or chronic carrier).

• case. 

in epidemiology, a countable instance in the population or study group of a particular disease, health disorder, or condition under investigation. sometimes, an individual with the particular disease.

• case-control study. 

a type of observational analytic study. enrollment into the study is based on presence (“case”) or absence (“control”) of disease. characteristics such as previous exposure are then compared between cases and controls.

• case definition. 

a set of standard criteria for deciding whether a person has a particular disease or health-related condition, by specifying clinical criteria and limitations on time, place, and person.

• case-fatality rate. 

the proportion of persons with a particular condition (cases) who die from that condition. the denominator is the number of incident cases; the numerator is the number of cause-specific deaths among those cases.

• cause of disease. 

a factor (characteristic, behavior, event, etc.) that directly influences the occurrence of disease. a reduction of the factor in the population should lead to a reduction in the occurrence of disease.

• cause-specific mortality rate. 

the mortality rate from a specified cause for a population. the numerator is the number of deaths attributed to a specific cause during a specified time interval; the denominator is the size of the population at the midpoint of the time interval.

• census. 

the enumeration of an entire population, usually with details being recorded on residence, age, sex, occupation, ethnic group, marital status, birth history, and relationship to head of household.

• chain of infection. 

a process that begins when an agent leaves its reservoir or host through a portal of exit, and is conveyed by some mode of transmission, then enters through an appropriate portal of entry to infect a susceptible host.

• class interval. 

a span of values of a continuous variable which are grouped into a single category for a frequency distribution of that variable.

• cluster. 

an aggregation of cases of a disease or other health-related condition, particularly cancer and birth defects, which are closely grouped in time and place. the number of cases may or may not exceed the expected number; frequently the expected number is not known.

• cohort. 

a well-defined group of people who have had a common experience or exposure, who are then followed up for the incidence of new diseases or events, as in a cohort or prospective study. a group of people born during a particular period or year is called a birth cohort.

• cohort study. 

a type of observational analytic study. enrollment into the study is based on exposure characteristics or membership in a group. disease, death, or other health-related outcomes are then ascertained and compared.

• common source outbreak. 

an outbreak that results from a group of persons being exposed to a common noxious influence, such as an infectious agent or toxin. if the group is exposed over a relatively brief period of time, so that all cases occur within one incubation period, then the common source outbreak is further classified as a point source outbreak. in some common source outbreaks, persons may be exposed over a period of days, weeks, or longer, with the exposure being either intermittent or continuous.

• confidence interval. 

a range of values for a variable of interest, e.g., a rate, constructed so that this range has a specified probability of including the true value of the variable. the specified probability is called the confidence level, and the end points of the confidence interval are called the confidence limits.

• confidence limit. 

the minimum or maximum value of a confidence interval.

• contact. 

exposure to a source of an infection, or a person so exposed.

• contagious. 

capable of being transmitted from one person to another by contact or close proximity.

• contingency table. 

a two-variable table with cross-tabulated data.

• control. 

in a case-control study, comparison group of persons without disease.

• crude mortality rate. 

the mortality rate from all causes of death for a population.

• cumulative frequency. 

in a frequency distribution, the number or proportion of cases or events with a particular value or in a particular class interval, plus the total number or proportion of cases or events with smaller values of the variable.

• cumulative frequency curve. 

a plot of the cumulative frequency rather than the actual frequency for each class interval of a variable. this type of graph is useful for identifying medians, quartiles, and other percentiles.  

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D

• death-to-case ratio. 

the number of deaths attributed to a particular disease during a specified time period divided by the number of new cases of that disease identified during the same time period.

• demographic information. 

the person: characteristics—age, sex, race, and occupation—of descriptive epidemiology used to characterize the populations at risk.

• denominator. 

the lower portion of a fraction used to calculate a rate or ratio. in a rate, the denominator is usually the population (or population experience, as in person-years, etc.) at risk.

• dependent variable. 

in a statistical analysis, the outcome variable(s) or the variable(s) whose values are a function of other variable(s) (called independent variable(s) in the relationship under study).

• descriptive epidemiology. 

the aspect of epidemiology concerned with organizing and summarizing health-related data according to time, place, and person.

• determinant. 

any factor, whether event, characteristic, or other definable entity, that brings about change in a health condition, or in other defined characteristics.

• direct transmission. 

the immediate transfer of an agent from a reservoir to a susceptible host by direct contact or droplet spread.

• distribution. 

in epidemiology, the frequency and pattern of health-related characteristics and events in a population. in statistics, the observed or theoretical frequency of values of a variable.

• dot plot. 

a visual display of the actual data points of a noncontinuous variable.

• droplet nuclei. 

the residue of dried droplets that may remain suspended in the air for long periods, may be blown over great distances, and are easily inhaled into the lungs and exhaled.

• droplet spread. 

the direct transmission of an infectious agent from a reservoir to a susceptible host by spray with relatively large, short-ranged aerosols produced by sneezing, coughing, or talking.  

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E

• endemic disease. 

the constant presence of a disease or infectious agent within a given geographic area or population group; may also refer to the usual prevalence of a given disease within such area or group.

• environmental factor. 

an extrinsic factor (geology, climate, insects, sanitation, health services, etc.) which affects the agent and the opportunity for exposure.

• epidemic. 

the occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time.

• epidemic curve. 

a histogram that shows the course of a disease outbreak or epidemic by plotting the number of cases by time of onset.

• epidemic period. 

a time period when the number of cases of disease reported is greater than expected.

• epidemiologic triad. 

the traditional model of infectious disease causation. includes three components: an external agent, a susceptible host, and an environment that brings the host and agent together, so that disease occurs.

• epidemiology. 

the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.

• evaluation. 

a process that attempts to determine as systematically and objectively as possible the relevance, effectiveness, and impact of activities in the light of their objectives.

• experimental study. 

a study in which the investigator specifies the exposure category for each individual (clinical trial) or community (community trial), then follows the individuals or community to detect the effects of the exposure.

• exposed (group). 

a group whose members have been exposed to a supposed cause of disease or health state of interest, or possess a characteristic that is a determinant of the health outcome of interest. 

F

• frequency distribution. 

a complete summary of the frequencies of the values or categories of a variable; often displayed in a two column table: the left column lists the individual values or categories, the right column indicates the number of observations in each category.

• frequency polygon. 

a graph of a frequency distribution with values of the variable on the x-axis and the number of observations on the y-axis; data points are plotted at the midpoints of the intervals and are connected with a straight line.  

G

• graph. 

a way to show quantitative data visually, using a system of coordinates.

H

• health. 

a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.

• health indicator. 

a measure that reflects, or indicates, the state of health of persons in a defined population, e.g., the infant mortality rate.

• health information system. 

a combination of health statistics from various sources, used to derive information about health status, health care, provision and use of services, and impact on health.

• high-risk group. 

a group in the community with an elevated risk of disease.

• histogram. 

a graphic representation of the frequency distribution of a continuous variable. rectangles are drawn in such a way that their bases lie on a linear scale representing different intervals, and their heights are proportional to the frequencies of the values within each of the intervals.

• host. 

a person or other living organism that can be infected by an infectious agent under natural conditions.

• host factor. 

an intrinsic factor (age, race, sex, behaviors, etc.) which influences an individual’s exposure, susceptibility, or response to a causative agent.

• hyperendemic disease. 

a disease that is constantly present at a high incidence and/or prevalence rate.

• hypothesis. 

a supposition, arrived at from observation or reflection, that leads to refutable predictions. any conjecture cast in a form that will allow it to be tested and refuted.

• hypothesis, null. 

the first step in testing for statistical significance in which it is assumed that the exposure is not related to disease.

• hypothesis, alternative. 

the hypothesis, to be adopted if the null hypothesis proves implausible, in which exposure is associated with disease.  

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I

• immunity, active. 

resistance developed in response to stimulus by an antigen (infecting agent or vaccine) and usually characterized by the presence of antibody produced by the host.

• immunity, herd. 

the resistance of a group to invasion and spread of an infectious agent, based on the resistance to infection of a high proportion of individual members of the group. the resistance is a product of the number susceptible and the probability that those who are susceptible will come into contact with an infected person.

• immunity, passive. 

immunity conferred by an antibody produced in another host and acquired naturally by an infant from its mother or artificially by administration of an antibody-containing preparation (antiserum or immune globulin).

• incidence rate. 

a measure of the frequency with which an event, such as a new case of illness, occurs in a population over a period of time. the denominator is the population at risk; the numerator is the number of new cases occurring during a given time period.

• incubation period. 

a period of subclinical or inapparent pathologic changes following exposure, ending with the onset of symptoms of infectious disease.

• independent variable. 

an exposure, risk factor, or other characteristic being observed or measured that is hypothesized to influence an event or manifestation (the dependent variable).

• indirect transmission. 

the transmission of an agent carried from a reservoir to a susceptible host by suspended air particles or by animate (vector) or inanimate (vehicle) intermediaries.

• individual data. 

data that have not been put into a frequency distribution or rank ordered.

• infectivity. 

the proportion of persons exposed to a causative agent who become infected by an infectious disease.

• inference, statistical. 

in statistics, the development of generalizations from sample data, usually with calculated degrees of uncertainty.

• interquartile range. 

the central portion of a distribution, calculated as the difference between the third quartile and the first quartile; this range includes about one-half of the observations in the set, leaving one-quarter of the observations on each side.  A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

J

None

K

None

L

• latency period. 

a period of subclinical or inapparent pathologic changes following exposure, ending with the onset of symptoms of chronic disease.  

M

• mean, arithmetic. 

the measure of central location commonly called the average. it is calculated by adding together all the individual values in a group of measurements and dividing by the number of values in the group.

• mean, geometric. 

the mean or average of a set of data measured on a logarithmic scale.

• measure of association. 

a quantified relationship between exposure and disease; includes relative risk, rate ratio, odds ratio.

• measure of central location. 

a central value that best represents a distribution of data. measures of central location include the mean, median, and mode. also called the measure of central tendency.

• measure of dispersion. 

a measure of the spread of a distribution out from its central value. measures of dispersion used in epidemiology include the interquartile range, variance, and the standard deviation.

• median. 

the measure of central location which divides a set of data into two equal parts.

• medical surveillance. 

the monitoring of potentially exposed individuals to detect early symptoms of disease.

• midrange. 

the halfway point or midpoint in a set of observations. for most types of data, it is calculated as the sum of the smallest observation and the largest observation, divided by two. for age data, one is added to the numerator. the midrange is usually calculated as an intermediate step in determining other measures.

• mode. 

a measure of central location, the most frequently occurring value in a set of observations.

• morbidity. 

any departure, subjective or objective, from a state of physiological or psychological well-being.

• mortality rate. 

a measure of the frequency of occurrence of death in a defined population during a specified interval of time.

• mortality rate, infant. 

a ratio expressing the number of deaths among children under one year of age reported during a given time period divided by the number of births reported during the same time period. the infant mortality rate is usually expressed per 1,000 live births.

• mortality rate, neonatal. 

a ratio expressing the number of deaths among children from birth up to but not including 28 days of age divided by the number of live births reported during the same time period. the neonatal mortality rate is usually expressed per 1,000 live births.

• mortality rate, postneonatal. 

a ratio expressing the number of deaths among children from 28 days up to but not including 1 year of age during a given time period divided by the number of lives births reported during the same time period. the postneonatal mortality rate is usually expressed per 1,000 live births. 

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N

• natural history of disease. 

the temporal course of disease from onset (inception) to resolution.

• necessary cause. 

a causal factor whose presence is required for the occurrence of the effect (of disease).

• nominal scale. 

classification into unordered qualitative categories; e.g., race, religion, and country of birth as measurements of individual attributes are purely nominal scales, as there is no inherent order to their categories.

• normal curve. 

a bell-shaped curve that results when a normal distribution is graphed.

• normal distribution. 

the symmetrical clustering of values around a central location. the properties of a normal distribution include the following: (1) it is a continuous, symmetrical distribution; both tails extend to infinity; (2) the arithmetic mean, mode, and median are identical; and, (3) its shape is completely determined by the mean and standard deviation.

• numerator. 

the upper portion of a fraction.

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O

• observational study. 

epidemiological study in situations where nature is allowed to take its course. changes or differences in one characteristic are studied in relation to changes or differences in others, without the intervention of the investigator.

• odds ratio. 

a measure of association which quantifies the relationship between an exposure and health outcome from a comparative study; also known as the cross-product ratio.

• ordinal scale. 

classification into ordered qualitative categories; e.g., social class (i, ii, iii, etc.), where the values have a distinct order, but their categories are qualitative in that there is no natural (numerical) distance between their positive values.

• outbreak. 

synonymous with epidemic. sometimes the preferred word, as it may escape sensationalism associated with the word epidemic. alternatively, a localized as opposed to generalized epidemic.  

P

• pandemic. 

an epidemic occurring over a very wide area (several countries or continents) and usually affecting a large proportion of the population.

• pathogenicity. 

the proportion of persons infected, after exposure to a causative agent, who then develop clinical disease.

• percentile. 

the set of numbers from 0 to 100 that divide a distribution into 100 parts of equal area, or divide a set of ranked data into 100 class intervals with each interval containing 1/100 of the observations. a particular percentile, say the 5th percentile, is a cut point with 5 percent of the observations below it and the remaining 95% of the observations above it.

• period prevalence. 

the amount a particular disease present in a population over a period of time.

• person-time rate. 

a measure of the incidence rate of an event, e.g., a disease or death, in a population at risk over an observed period to time, that directly incorporates time into the denominator.

• pie chart. 

a circular chart in which the size of each “slice” is proportional to the frequency of each category of a variable

• point prevalence. 

the amount of a particular disease present in a population at a single point in time.

• population. 

the total number of inhabitants of a given area or country. in sampling, the population may refer to the units from which the sample is drawn, not necessarily the total population of people.

• predictive value positive. 

a measure of the predictive value of a reported case or epidemic; the proportion of cases reported by a surveillance system or classified by a case definition which are true cases.

• prevalence. 

the number or proportion of cases or events or conditions in a given population.

• prevalence rate. 

the proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time.

• propagated outbreak. 

an outbreak that does not have a common source, but instead spreads from person to person.

• proportion. 

a type of ratio in which the numerator is included in the denominator. the ratio of a part to the whole, expressed as a “decimal fraction” (e.g., 0.2), as a fraction (1/5), or, loosely, as a percentage (20%).

• proportionate mortality. 

the proportion of deaths in a specified population over a period of time attributable to different causes. each cause is expressed as a percentage of all deaths, and the sum of the causes must add to 100%. these proportions are not mortality rates, since the denominator is all deaths, not the population in which the deaths occurred.

• public health surveillance. 

the systematic collection, analysis, interpretation, and dissemination of health data on an ongoing basis, to gain knowledge of the pattern of disease occurrence and potential in a community, in order to control and prevent disease in the community.  

Q

none

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R

• race-specific mortality rate. 

a mortality rate limited to a specified racial group. both numerator and denominator are limited to the specified group.

• random sample. 

a sample derived by selecting individuals such that each individual has the same probability of selection.

• range. 

in statistics, the difference between the largest and smallest values in a distribution. in common use, the span of values from smallest to largest.

• rate. 

an expression of the frequency with which an event occurs in a defined population.

• rate ratio. 

a comparison of two groups in terms of incidence rates, person-time rates, or mortality rates.

• ratio. 

the value obtained by dividing one quantity by another.

• relative risk. 

a comparison of the risk of some health-related event such as disease or death in two groups.

• representative sample. 

a sample whose characteristics correspond to those of the original population or reference population.

• reservoir. 

the habitat in which an infectious agent normally lives, grows and multiplies; reservoirs include human reservoirs, animals reservoirs, and environmental reservoirs.

• risk. 

the probability that an event will occur, e.g. that an individual will become ill or die within a stated period of time or age.

• risk factor. 

an aspect of personal behavior or lifestyle, an environmental exposure, or an inborn or inherited characteristic that is associated with an increased occurrence of disease or other health-related event or condition.

• risk ratio. 

a comparison of the risk of some health-related event such as disease or death in two groups.  

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S

• sample. 

a selected subset of a population. a sample may be random or non-random and it may be representative or non-representative.

• scatter diagram. 

a graph in which each dot represents paired values for two continuous variables, with the x-axis representing one variable and the y-axis representing the other; used to display the relationship between the two variables; also called a scattergram.

• seasonality. 

change in physiological status or in disease occurrence that conforms to a regular seasonal pattern.

• secondary attack rate. 

a measure of the frequency of new cases of a disease among the contacts of known cases.

• secular trend. 

changes over a long period of time, generally years or decades.

• sensitivity. 

the ability of a system to detect epidemics and other changes in disease occurrence. the proportion of persons with disease who are correctly identified by a screening test or case definition as having disease.

• sentinel surveillance. 

a surveillance system in which a pre-arranged sample of reporting sources agrees to report all cases of one or more notifiable conditions.

• sex-specific mortality rate. 

a mortality rate among either males or females.

• skewed. 

a distribution that is asymmetrical.

• specificity. 

the proportion of persons without disease who are correctly identified by a screening test or case definition as not having disease.

• sporadic. 

a disease that occurs infrequently and irregularly.

• spot map. 

a map that indicates the location of each case of a rare disease or outbreak by a place that is potentially relevant to the health event being investigated, such as where each case lived or worked.

• standard deviation. 

the most widely used measure of dispersion of a frequency distribution, equal to the positive square root of the variance.

• standard error (of the mean). 

the standard deviation of a theoretical distribution of sample means about the true population mean.

• sufficient cause. 

a causal factor or collection of factors whose presence is always followed by the occurrence of the effect (of disease).

• surveillance. 

see 

• public health surveillance

• survival curve. 

a curve that starts at 100% of the study population and shows the percentage of the population still surviving at successive times for as long as information is available. may be applied not only to survival as such, but also to the persistence of freedom from a disease, or complication or some other endpoint.  A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

T

• table. 

a set of data arranged in rows and columns.

• table shell. 

a table that is complete except for the data.

• transmission of infection. 

any mode or mechanism by which an infectious agent is spread through the environment or to another person.

• trend. 

a long-term movement or change in frequency, usually upwards or downwards. ==U==

• universal precautions. 

recommendations issued by cdc to minimize the risk of transmission of bloodborne pathogens, particularly hiv and hbv, by health care and public safety workers. barrier precautions are to be used to prevent exposure to blood and certain body fluids of all patients.  

V

• validity. 

the degree to which a measurement actually measures or detects what it is supposed to measure.

• variable. 

any characteristic or attribute that can be measured.

• variance. 

a measure of the dispersion shown by a set of observations, defined by the sum of the squares of deviations from the mean, divided by the number of degrees of freedom in the set of observations.

• vector. 

an animate intermediary in the indirect transmission of an agent that carries the agent from a reservoir to a susceptible host.

• vehicle. 

an inanimate intermediary in the indirect transmission of an agent that carries the agent from a reservoir to a susceptible host.

• virulence. 

the proportion of persons with clinical disease, who after becoming infected, become severely ill or die.

• vital statistics. 

systematically tabulated information about births, marriages, divorces, and deaths, based on registration of these vital events.

W

none

X

none

Y

• years of potential life lost. 

a measure of the impact of premature mortality on a population, calculated as the sum of the differences between some predetermined minimum or desired life span and the age of death for individuals who died earlier than that predetermined age.

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Z

• zoonoses. 

an infectious disease that is transmissible under normal conditions from animals to humans.

More terms

• active immunity see  immunity, active.

• active surveillance see  surveillance, active.

• age-adjusted mortality rate see  mortality rate, age-adjusted.

• agent a factor (e.g., a microorganism or chemical substance) or form of energy whose presence, excessive presence, or in the case of deficiency diseases, relative absence is essential for the occurrence of a disease or other adverse health outcome.

• age-specific mortality rate see  mortality rate, age-specific.

• alternative hypothesis see  hypothesis, alternative.

• analytic epidemiology see  epidemiology, analytic.

• analytic study see  study, analytic.

• antibody any of a variety of proteins in the blood that are produced in response to an antigen as an immune response.

• antigen any substance (e.g., a toxin or the surface of a microorganism or transplanted organ) recognized as foreign by the human body and that stimulates the production of antibodies.

• applied epidemiology see  epidemiology, applied.

• arbovirus any of a group of viruses that are transmitted between hosts by mosquitoes, ticks, and other arthropods.

• arithmetic mean see  mean, arithmetic.

• arithmetic-scale line graph see  line graph, arithmetic-scale.

• arthropod an organism that has jointed appendages and segmented external skeleton (e.g., flies, mosquitoes, ticks, or mites).

• association the statistical relation between two or more events, characteristics, or other variables.

• asymmetrical a type of distribution where the shape to the right and left of the central location is not the same. Often referred to as a skewed distribution; the mean, median, and mode of an asymmetrical distribution are not the same.

• asymmetrical without symptoms.

• attack rate a form of incidence that measures the proportion of persons in a population who experience an acute health event during a limited period (e.g., during an outbreak), calculated as the number of new cases of a health problem during an outbreak divided by the size of the population at the beginning of the period, usually expressed as a percentage or per 1,000 or 100,000 population (see also incidence proportion).

• attack rate, secondary a measure of the frequency of new cases of a disease among the contacts of known patients.

• attributable proportion see  proportion, attributable.

• attributable risk percent see  proportion, attributable.

• attribute a risk factor that is an intrinsic characteristic of the individual person, animal, plant, or other type of organism under study (e.g., genetic susceptibility, age, sex, breed, weight).

• axis one of the dimensions of a graph in a rectangular graph, the x-axis is the horizontal axis, and the y-axis is the vertical axis.

• bar chart a visual display in which each category of a variable is represented by a bar or column bar charts are used to illustrate variations in size among categories.

• bar chart, 100% component a stacked bar chart in which all bars or columns are the same length, and the measured axis represents 0%–100%.

• bar chart, deviation a bar chart displaying either positive or negative differences from a baseline.

• bar chart, grouped a bar chart displaying quantities of two variables, represented by adjoining bars or columns (i.e., a group) of categories of one variable, separated by space between groups.

• bar chart, stacked a bar chart displaying quantities of two variables, represented by subdivided bars or columns (the subdivisions representing the categories of one variable) separated by space between bars or columns.

• bias a systematic deviation of results or inferences from the truth or processes leading to such systematic deviation; any systematic tendency in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that are systematically different from the truth. In epidemiology, does not imply intentional deviation.

• bias, information systematic difference in the collection of data regarding the participants in a study (e.g., about exposures in a case-control study, or about health outcomes in a cohort study) that leads to an incorrect result (e.g., risk ratio or odds ratio) or inference.

• bias, selection systematic difference in the enrollment of participants in a study that leads to an incorrect result (e.g., risk ratio or odds ratio) or inference.

• bimodal having two data peaks.

• biologic transmission see  transmission, biologic.

• birth cohort see  cohort, birth.

• birth rate, crude the number of live births during a specified period divided by the mid-period population, usually expressed per 1,000 population.

• box plot a visual display that summarizes data by using a “box and whiskers” format to indicate the minimum and maximum values (ends of the whiskers), interquartile range (length of the box), and median (line through the box).

• carrier a person or animal that harbors the infectious agent for a disease and can transmit it to others, but does not demonstrate signs of the disease. A carrier can be asymptomatic (never indicate signs of the disease) or can display signs of the disease only during the incubation period, convalescence, or postconvalescence. The period of being a carrier can be short (a transient carrier) or long (a chronic carrier).

• case an instance of a particular disease, injury, or other health conditions that meets selected criteria (see also case definition). Using the term to describe the person rather than the health condition is discouraged (see also case-patient).

• case-control study see  study, case-control.

• case definition a set of uniformly applied criteria for determining whether a person should be identified as having a particular disease, injury, or other health condition. In epidemiology, particularly for an outbreak investigation, a case definition specifies clinical criteria and details of time, place, and person.

• case-fatality rate (also called * case-fatality ratio) the proportion of persons with a particular condition (e.g., patients) who die from that condition. The denominator is the number of persons with the condition; the numerator is the number of cause-specific deaths among those persons.

• case, index the first case or instance of a patient coming to the attention of health authorities.

• case-patient in a case-control study, a person who has the disease, injury, or other health condition that meets the case definition (see also case).

• case, source the case or instance of a patient responsible for transmitting infection to others; the instance of a patient who gives rise to an outbreak or epidemic.

• cause, component a factor that contributes to a sufficient cause (see also cause, sufficient).

• cause of disease a factor (e.g., characteristic, behavior, or event) that directly influences the occurrence of a disease. Reducing such a factor among a population should reduce occurrence of the disease.

• cause, necessary a factor that must be present for a disease or other health problem to occur.

• cause-specific mortality rate see  mortality rate, cause-specific.

• cause, sufficient a factor or collection of factors whose presence is always followed by the occurrence of a particular health problem.

• census the enumeration of an entire population, usually including details on residence, age, sex, occupation, racial/ethnic group, marital status, birth history, and relationship to the head of household.

• central location (also called * central tendency) a statistical measurement to quantify the middle or the center of a distribution. Of the multiple ways to define central tendency, the most common are the mean, median, and mode.

• chain of infection the progression of an infectious agent that leaves its reservoir or host through a portal of exit, is conveyed by a mode of transmission, and then enters through an appropriate portal of entry to infect a susceptible host.

• “chartjunk” unnecessary or confusing visual elements in charts, illustrations, or graphs. The term was first used by Edward Tufte in his book, The Visual Display of Quantitative Information (1983).

• class interval the span of values of a continuous variable that are grouped into a single category (see also class), usually to create a frequency distribution for that variable.

• class limits the values at the upper and lower ends of a class interval.

• clinical criteria the medical features (e.g., symptoms, medical examination findings, and laboratory results) that are used in a case definition.

• clinical disease a disease that has been manifested by its symptoms and features.

• clinical trial see  trial, clinical.

• cluster an aggregation of cases of a disease, injury, or other health condition (particularly cancer and birth defects) in a circumscribed area during a particular period without regard to whether the number of cases is more than expected (often the expected number is not known).

• cohort a well-defined group of persons who have had a common experience or exposure and are then followed up, as in a cohort study or prospective study, to determine the incidence of new diseases or health events.

• cohort, birth a group of persons born during a particular period or year.

• cohort study see  study, cohort.

• common-source outbreak see  outbreak, common-source.

• community immunity see  immunity, herd.

• community trial see  trial, community.

• comparison group a group in an analytic study (e.g., a cohort or case-control study) with whom the primary group of interest (exposed group in a cohort study or case-patients in a case-control study) is compared. The comparison group provides an estimate of the background or expected incidence of disease (in a cohort study) or exposure (in a case-control study).

• confidence interval a range of values for a measure (e.g., rate or odds ratio) constructed so that the range has a specified probability (often, but not necessarily, 95%) of including the true value of the measure.

• confidence limits the end points (i.e., the minimum and maximum values) of a confidence.

• confounding the distortion of the association between an exposure and a health outcome by a third variable that is related to both.

• contact exposure to a source of an infection; a person who has been exposed.

• contact, direct exposure or transmission of an agent from a source to a susceptible host through touching (e.g., from a human host by kissing, sexual intercourse, or skin-to-skin contact) or from touching an infected animal or contaminated soil or vegetation.

• contagious capable of being transmitted from one person to another by contact or close proximity.

• contingency table a two-variable table of cross-tabulated data.

• continuous variable see  variable, continuous.

• control in a case-control study, a member of the group of persons without the health problem under study (see also comparison group and study, case-control).

• crude when referring to a rate, an overall or summary rate for a population, without adjustment.

• crude birth rate see  birth rate, crude.

• crude death rate see  mortality rate, crude.

• crude mortality rate see  mortality rate, crude.

• cumulative frequency in a frequency distribution, the number or proportion of observations with a particular value and any smaller value.

• cumulative frequency curve a plot of the cumulative frequency rather than the actual frequency for each class interval of a variable. This type of graph is useful for identifying medians and quartiles and other percentiles.

• death-to-case ratio the number of deaths attributed to a particular disease, injury, or other health condition during a specified period, divided by the number of new cases of that disease, injury, or condition identified during the same period.

• decision analysis application of quantitative methods to decision-making.

• decision tree a branching chart that represents the logical sequence or pathway of a clinical or public health decision.

• demographic information personal characteristics of a person or group (e.g., age, sex, race/ethnicity, residence, and occupation) demographic information is used in descriptive epidemiology to characterize patients or populations.

• dendrogram see  phylogenetic tree.

• denominator the lower portion of a fraction; used in calculating a ratio, proportion, or rate. For a rate, the denominator is usually the midinterval population.

• dependent variable see  variable, dependent.

• descriptive epidemiology see  epidemiology, descriptive.

• determinant any factor that brings about change in a health condition or in other defined characteristics (see also cause and risk factor).

• direct transmission see  transmission, direct.

• discrete variable (or * data) see variable (or data), discrete.

• distribution in epidemiology, the frequency and pattern of health-related characteristics and events in a population. In statistics, the frequency and pattern of the values or categories of a variable.

• dose-response association between an exposure and health outcome that varies in a consistently increasing or decreasing fashion as the amount of exposure (dose) increases.

• dot plot a visual display of the specific data points of a variable.

• droplet nuclei the residue of dried droplets of infectious agents that is easily inhaled and exhaled and can remain suspended in air for relatively long periods or be blown over great distances.

• droplet spread the direct transmission of an infectious agent by means of the aerosols produced in sneezing, coughing, or talking that travel only a short distance before falling to the ground.

• effect the result of a cause.

• effectiveness the ability of an intervention or program to produce the intended or expected results in the field.

• efficacy the ability of an intervention or program to produce the intended or expected results under ideal conditions.

• efficiency the ability of an intervention or program to produce the intended or expected results with a minimum expenditure of time and resources.

• EIS Epidemic Intelligence Service; CDC’s 2-year training program in applied epidemiology for public health professionals (/eis).

• endemic the constant presence of an agent or health condition within a given geographic area or population; can also refer to the usual prevalence of an agent or condition.

• environmental factor an extrinsic factor (e.g., geology, climate, insects, sanitation, or health services) that affects an agent and the opportunity for exposure.

• epidemic the occurrence of more cases of disease, injury, or other health condition than expected in a given area or among a specific group of persons during a particular period. Usually, the cases are presumed to have a common cause or to be related to one another in some way (see also outbreak).

• epidemic curve a histogram that displays the course of an outbreak or epidemic by plotting the number of cases according to time of onset.

• epidemic period the time span of an outbreak or epidemic.

• epidemiologic triad the traditional model of infectious disease causation having three components: an external agent, a susceptible host, and an environment that brings the host and agent together so that disease occurs.

• epidemiology the study of the distribution and determinants of health conditions or events among populations and the application of that study to control health problems.

• epidemiology, analytic the aspect of epidemiology concerned with why and how a health problem occurs. Analytic epidemiology uses comparison groups to provide baseline or expected values so that associations between exposures and outcomes can be quantified and hypotheses about the cause of the problem can be tested (see also study, analytic).

• epidemiology, applied the application or practice of epidemiology to control and prevent health problems.

• epidemiology, descriptive the aspect of epidemiology concerned with organizing and summarizing data regarding the persons affected (e.g., the characteristics of those who became ill), time (e.g., when they become ill), and place (e.g., where they might have been exposed to the cause of illness).

• epidemiology, field applied epidemiology (i.e., the application or practice of epidemiology to control and prevent health problems), particularly when the epidemiologist(s) must travel to and work in the community in which the health problem is occurring or has occurred.

• evaluation systematic and objective examination of activities to determine their relevance, effectiveness, and impact.

• excess risk risk difference, calculated as the risk among the exposed group minus the risk among the unexposed group.

• experimental study see  study, experimental.

• exposed group a group whose members have had contact with a suspected cause of, or possess a characteristic that is a suspected determinant of, a particular health problem.

• exposure having come into contact with a cause of, or possessing a characteristic that is a determinant of, a particular health problem.

• false-negative a negative test result for a person who actually has the condition similarly, a person who has the disease (perhaps mild or variant) but who does not fit the case definition, or a patient or outbreak not detected by a surveillance system.

• false-positive a positive test result for a person who actually does not have the condition. Similarly, a person who does not have the disease but who nonetheless fits the case definition, or a patient or outbreak erroneously identified by a surveillance system.

• field epidemiology see  epidemiology, field.

• follow-up study see  study, cohort.

• fomite an inanimate object that can be the vehicle for transmission of an infectious agent (e.g., bedding, towels, or surgical instruments).

• forest plot a graph that displays the point estimates and confidence intervals of individual studies included in a meta-analysis or systematic review as a series of parallel lines.

• frequency the amount or number of occurrences of an attribute or health outcome among a population.

• frequency distribution a complete summary of the frequencies of the values or categories of a variable, often displayed in a two-column table with the individual values or categories in the left column and the number of observations in each category in the right column.

• frequency polygon a graph of a frequency distribution in which values of the variable are plotted on the horizontal axis, and the number of observations are plotted on the vertical axis. Data points are plotted at the midpoints of the intervals and are connected with straight lines.

• geometric mean see  mean, geometric.

• graph a visual display of quantitative data arranged on a system of coordinates.

• health a state of complete physical, mental, and social well-being and not merely the absence of disease or other infirmity.

• health indicator any of a variety of measures (e.g., mortality rate) that indicate the state of health of a given population.

• health information system a combination of health statistics from different sources. Data from these systems are used to learn about health status, health care, provision and use of services, and the impact of services and programs on health.

• healthy worker effect the observation that employed persons generally have lower mortality rates than the general population, because persons with severe, disabling disease (who have higher mortality rates) tend to be excluded from the workforce.

• herd immunity see  immunity, herd.

• high-risk group a group of persons whose risk for a particular disease, injury, or other health condition is greater than that of the rest of their community or population.

• HIPAA the Health Insurance Portability and Accountability Act, enacted in 1996, which addresses the privacy of a person’s medical information as well as postemployment insurance and other health-related concerns.

• histogram a visual representation of the frequency distribution of a continuous variable. The class intervals of the variable are grouped on a linear scale on the horizontal axis, and the class frequencies are grouped on the vertical axis. Columns are drawn so that their bases equal the class intervals (i.e., so that columns of adjacent intervals touch), and their heights correspond to the class frequencies.

• host a person or other living organism that is susceptible to or harbors an infectious agent under natural conditions.

• host factor an intrinsic factor (e.g., age, race/ethnicity, sex, or behaviors) that influences a person’s exposure, susceptibility, or response to an agent.

• hyperendemic the constant presence at high incidence and prevalence of an agent or health condition within a given geographic area or population.

• hypothesis a supposition, arrived at from observation or reflection, that leads to refutable predictions; any conjecture cast in a form that will allow it to be tested and refuted.

• hypothesis, alternative the supposition that an exposure is associated with the health condition under study. The alternative is adopted if the null hypothesis (see also hypothesis, null) proves implausible.

• hypothesis, null the supposition that two (or more) groups do not differ in the measure of interest (e.g., incidence or proportion exposed); the supposition that an exposure is not associated with the health condition under study, so that the risk ratio or odds ratio equals 1. The null hypothesis is used in conjunction with statistical testing.

• immunity, active resistance developed in response to an antigen (i.e., an infecting agent or vaccine), usually characterized by the presence of antibody produced by the host.

• immunity, herd the resistance to an infectious agent of an entire group or community (and, in particular, protection of susceptible persons) as a result of a substantial proportion of the population being immune to the agent. Herd immunity is based on having a substantial number of immune persons, thereby reducing the likelihood that an infected person will come in contact with a susceptible one among human populations, also called * community immunity.

• immunity, passive immunity conferred by an antibody produced in another host This type of immunity can be acquired naturally by an infant from its mother or artificially by administration of an antibody-containing preparation (e.g., antiserum or immune globulin).

• incidence a measure of the frequency with which new cases of illness, injury, or other health condition occurs among a population during a specified period.

• incidence proportion the fraction of persons with new cases of illness, injury, or other health condition during a specified period, calculated as the number of new cases divided by the size of the population at the start of the study period (see also attack rate).

• incidence rate a measure of the frequency with which new cases of illness, injury, or other health condition occur, expressed explicitly per a time frame. Incidence rate is calculated as the number of new cases over a specified period divided either by the average population (usually mid-period) or by the cumulative person-time the population was at risk.

• incubation period the time interval from exposure to an infectious agent to the onset of symptoms of an infectious disease.

• independent variable see  variable, independent.

• index case see  case, index.

• indirect transmission see  transmission, indirect.

• individual data values or observations from each record (also called raw data).

• infant mortality rate see  mortality rate, infant.

• infection invasion of the body tissues of a host by an infectious agent, whether or not it causes disease.

• infectivity the ability of an infectious agent to cause infection, measured as the proportion of persons exposed to an infectious agent who become infected.

• information bias see  bias, information.

• interquartile range a measure of spread representing the middle 50% of the observations, calculated as the difference between the third quartile (75th percentile) and the first quartile (25th percentile).

• isolation the separation of infected persons to prevent transmission to susceptible ones. Isolation refers to separation of ill persons; * quarantine refers to separation of potentially exposed but well persons.

• latency period the time from exposure to a causal agent to onset of symptoms of a (usually noninfectious) disease (see also incubation period).

• life expectancy a statistical projection of the average number of years a person of a given age is expected to live, if current mortality rates continue to apply.

• line graph, arithmetic-scale a graph that displays patterns or trends by plotting the frequency (e.g., number, proportion, or rate) of a characteristic or event during some variable, usually time. The y-axis, measuring frequency, uses an arithmetic scale.

• line graph, semilogarithmic-scale a graph that displays patterns or trends by plotting the frequency (e.g., number, proportion, or rate) of a characteristic or event during some variable, usually time. The y-axis, measuring frequency, uses a logarithmic scale.

• line listing a type of epidemiologic database, organized similar to a spreadsheet with rows and columns in which information from cases or patients are listed each column represents a variable, and each row represents an individual case or patient.

• logarithmic transformation conversion of nominal or ordinal data to logarithmic data. The purpose is to examine rate of change instead of amount of change only.

• map, area (shaded, choropleth) a visual display of the geographic pattern of a health problem, in which a marker is placed on a map to indicate where each affected person lives, works, or might have been exposed.

• mean (or average) commonly called the average; it is the most common measure of central tendency.

• mean, arithmetic the measure of central location, commonly called the average, calculated by adding all the values in a group of measurements and dividing by the number of values in the group.

• mean, geometric the mean, or average, of a set of data measured on a logarithmic scale.

• measure of association a quantified relationship between exposure and a particular health problem (e.g., risk ratio, rate ratio, and odds ratio).

• measure of central location a central value that best represents a distribution of data. Common measures of central location are the mean, median, and mode also called the measure of central tendency.

• measure of dispersion see  measure of spread.

• measure of spread a measure of the distribution of observations out from its central value. Measures of spread used in epidemiology include the interquartile range, variance, and the standard deviation.

• measurement scale the complete range of possible values for a measurement.

• mechanical transmission see  transmission, mechanical.

• median the measure of central location that divides a set of data into two equal parts, above and below which lie an equal number of values (see also measure of central location).

• medical surveillance see  surveillance, medical.

• midrange the halfway point, or midpoint, in a set of observations. For the majority of data, the midrange is calculated by adding the smallest observation and the largest observation and dividing by two. The midrange is usually calculated as an intermediate step in determining other measures.

• mode the most frequently occurring value in a set of observations (see also measure of central location).

• mode of transmission the manner in which an agent is transmitted from its reservoir to a susceptible host (see also transmission).

• morbidity disease; any departure, subjective or objective, from a state of physiological or psychological health and well-being.

• mortality death.

• mortality rate a measure of the frequency of occurrence of death among a defined population during a specified time interval.

• mortality rate, age-adjusted a mortality rate that has been statistically modified to eliminate the effect of different age distributions among different populations.

• mortality rate, age-specific a mortality rate limited to a particular age group, calculated as the number of deaths among the age group divided by the number of persons in that age group, usually expressed per 100,000.

• mortality rate, cause-specific the mortality rate from a specified cause, calculated as the number of deaths attributed to a specific cause during a specified time interval among a population divided by the size of the midinterval population.

• mortality rate, crude a mortality rate from all causes of death for an entire population, without adjustment.

• mortality rate, infant the mortality rate for children aged <1 year, calculated as the number of deaths reported among this age group during a given period divided by the number of live births reported during the same period, and expressed per 1,000 live births. Infant mortality rate is a universally accepted indicator of the health of a nation’s population and the adequacy of its health-care system.

• mortality rate, neonatal the mortality rate for children from age birth up to, but not including, 28 days. In calculating neonatal mortality rates, the numerator is the number of deaths among this age group during a given period, and the denominator is the number of live births reported during the same period. The neonatal mortality rate is usually expressed per 1,000 live births.

• mortality rate, postneonatal the mortality rate for children from age 28 days up to, but not including, 1 year. In calculating postneonatal mortality rates, the numerator is the number of deaths among this age group during a given period, and the denominator is the number of live births during the same period.. The postneonatal mortality rate is usually expressed per 1,000 live births.

• mortality rate, race/ethnic-specific a mortality rate limited to a specified racial or ethnic group both numerator and denominator are limited to that group.

• mortality rate, sex-specific a mortality rate among either males or females.

• natural history of disease the progression of a disease process in a person from the time it begins to the time it resolves, in the absence of treatment.

• NCHS The National Center for Health Statistics, the US governmental organization responsible for national vital statistics and multiple national health surveys. Organizationally, NCHS is a component of the Centers for Disease Control and Prevention, one of the agencies of the US Department of Health and Human Services.

• NHANES The National Health and Nutrition Examination Survey, a representative survey of the civilian, noninstitutionalized US population conducted by the National Center for Health Statistics, designed to (1) estimate the proportion of the US population and designated groups with selected disease and risk factors; (2) monitor trends in selected behaviors, exposures, and diseases; and (3) study the associations among diet, nutrition, and health.

• necessary cause see  cause, necessary.

• neonatal mortality rate see  mortality rate, neonatal.

• nominal scale see  scale, nominal.

• normal curve the bell-shaped curve that results when a normal distribution is graphed.

• normal distribution a distribution represented as a bell shape, symmetrical on both sides of the peak, which is simultaneously the mean, median, and mode, and with both tails extending to infinity.

• notifiable disease a disease that, by law, must be reported to public health authorities upon diagnosis.

• null hypothesis see  hypothesis, null.

• numerator the upper portion of a fraction (see also denominator).

• observational study see  study, observational.

• odds ratio a measure of association used in comparative studies, particularly case-control studies, that quantifies the association between an exposure and a health outcome; also called the cross-product ratio.

• ordinal scale see  scale, ordinal.

• outbreak the occurrence of more cases of disease, injury, or other health condition than expected in a given area or among a specific group of persons during a specific period. Usually, the cases are presumed to have a common cause or to be related to one another in some way. Sometimes distinguished from an epidemic as more localized, or the term less likely to evoke public panic (see also epidemic).

• outbreak, common-source an outbreak that results from persons being exposed to the same harmful influence (e.g., an infectious agent or toxin). The exposure period can be brief or can extend over days, weeks, or longer, with the exposure being either intermittent or continuous.

• outbreak, point-source a common source outbreak in which the exposure period is relatively brief so that all cases occur within one incubation period.

• outbreak, propagated an outbreak that spreads from person to person rather than from a common source.

• outcome(s) any or all of the possible results that can stem from exposure to a causal factor or from preventive or therapeutic interventions; all identified changes in health status that result from the handling of a health problem.

• outlier a value substantively or statistically different from all (or approximately all) of the other values in a distribution.

• P value the probability of observing an association between two variables or a difference between two or more groups as large or larger than that observed, if the null hypothesis were true. Used in statistical testing to evaluate the plausibility of the null hypothesis (i.e., whether the observed association or difference plausibly might have occurred by chance).

• pandemic an epidemic occurring over a widespread area (multiple countries or continents) and usually affecting a substantial proportion of the population.

• passive immunity see  immunity, passive.

• passive surveillance see  surveillance, passive.

• pathogenicity the ability of an agent to cause disease after infection, measured as the proportion of persons infected by an agent who then experience clinical disease.

• percentile a set of cut points used to divide a distribution or a set of ranked data into 100 parts of equal area with each interval between the points containing 1/100 or 1% of the observations. For example, the 5th percentile is a cut point with 5% of the observations below it and the remaining 95% above it.

• period prevalence see  prevalence, period.

• person-time rate the incidence rate calculated as the number of new cases among a population divided by the cumulative person-time of that population, usually expressed as the number of events per persons per unit of time.

• person-time the amount of time each participant in a cohort study is observed and disease-free, often summed to provide the denominator for a person-time rate.

• phylogenetic tree a branching chart that indicates the evolutionary lineage or genetic relatedness of organisms.

• pie chart a circular graph of a frequency distribution in which each segment of the pie is proportional in size to the frequency of corresponding category.

• point prevalence see  prevalence, point.

• point-source outbreak see  outbreak, point-source.

• population the total number of inhabitants of a geographic area or the total number of persons in a particular group (e.g., the number of persons engaged in a certain occupation).

• population pyramid a graphical display of the age-sex distribution of a population, constructed with a horizontal histogram of the age distribution of males pointing to the left, and the corresponding horizontal histogram of age distribution of females pointing to the right.

• portal of entry a pathway into the host that gives an agent access to tissue that will allow it to multiply or act.

• portal of exit a pathway by which an agent can leave its host.

• postneonatal mortality rate see  mortality rate, postneonatal.

• predictive value positive the proportion of cases identified by a test, reported by a surveillance system, or classified by a case definition that are true cases, calculated as the number of true-positives divided by the number of true-positives plus false-positives.

• prevalence the number or proportion of cases or events or attributes among a given population.

• prevalence rate the proportion of a population that has a particular disease, injury, other health condition, or attribute at a specified point in time (point prevalence) or during a specified period (period prevalence).

• prevalence, period the amount of a particular disease, chronic condition, or type of injury present among a population at any time during a particular period.

• prevalence, point the amount of a particular disease, chronic condition, or type of injury present among a population at a single point in time.

• privacy rule a set of regulations based on the Health Insurance Portability and Accountability Act to protect the privacy of individually identifiable health information.

• propagated outbreak see  outbreak, propagated.

• proportion a ratio in which the numerator is included in the denominator; the ratio of a part to the whole, expressed as a “decimal fraction” (e.g., 0 2), a fraction (1/5), or a percentage (20%).

• proportion, attributable a measure of the impact of a causative factor on the public health; the proportion of a health state or event among exposed persons that can be attributed to the exposure also called attributable risk percent.

• proportionate mortality the proportion of deaths among a population attributable to a particular cause during a selected period. Each cause of death is expressed as a percentage of all deaths, and the sum of the proportionate mortality for all causes must equal 100%. These proportions are not mortality rates because, in proportionate mortality, the denominator is all deaths instead of the population among whom the deaths occurred.

• prospective study see  study, prospective.

• quarantine the separation of well persons who have been exposed or are suspected to have been exposed to a communicable disease, to monitor for illness and to prevent potential transmission of infection to susceptible persons during the incubation period. Quarantine refers to separation of potentially exposed but well persons; * isolation refers to separation of ill persons.

• race/ethnic-specific mortality rate see  mortality rate, race/ethnic-specific.

• random sample see  sample, random.

• range in statistics, the difference between the largest and smallest values in a distribution; in common use, the span of values from smallest to largest.

• rate an expression of the relative frequency with which an event occurs among a defined population per unit of time, calculated as the number of new cases or deaths during a specified period divided by either person-time or the average (midinterval) population. In epidemiology, it is often used more casually to refer to proportions that are not truly rates (e.g., attack rate or case-fatality rate).

• rate ratio a measure of association that quantifies the relation between an exposure and a health outcome from an epidemiologic study, calculated as the ratio of incidence rates or mortality rates of two groups.

• ratio the relative size of two quantities, calculated by dividing one quantity by the other.

• record in a line listing, each row is a record or observation. A record represents data related to a single case.

• relative risk a general term for measures of association calculated from the data in a two-by-two table, including risk ratio, rate ratio, and odds ratio (see also risk ratio).

• representative sample see  sample, representative.

• reservoir the habitat in which an infectious agent normally lives, grows, and multiplies, which can include humans, animals, or the environment.

• retrospective study see  study, retrospective.

• risk the probability that an event will occur (e.g., that a person will be affected by, or die from, an illness, injury, or other health condition within a specified time or age span).

• risk factor an aspect of personal behavior or lifestyle, an environmental exposure, or a hereditary characteristic that is associated with an increase in the occurrence of a particular disease, injury, or other health condition.

• risk ratio a measure of association that quantifies the association between an exposure and a health outcome from an epidemiologic study, calculated as the ratio of incidence proportions of two groups.

• sample a selected subset of a population a sample can be random or nonrandom and representative or nonrepresentative.

• sample, random a sample of persons chosen in such a way that each one has the same (and known) probability of being selected.

• sample, representative a sample whose characteristics correspond to those of the original or reference population.

• scale, interval a measurement scale consisting of quantitative categories whose values are measured on a scale of equally spaced units, but without a true zero point (e.g., date of birth).

• scale, nominal a measurement scale consisting of qualitative categories whose values have no inherent statistical order or rank (e.g., categories of race/ethnicity, religion, or country of birth).

• scale, ordinal a measurement scale consisting of qualitative categories whose values have a distinct order but no numerical distance between their possible values (e.g., stage of cancer, I, II, III, or IV).

• scale, ratio a measurement scale consisting of quantitative categories whose values are intervals with a true zero point (e.g., height in centimeters or duration of illness).

• scatter diagram (or * scattergram) a graphical display of the association between two variables in which a dot is plotted on the graph for each set of paired values for two continuous variables, with one variable plotted on the horizontal axis, and the other plotted on the vertical axis.

• seasonality change in physiologic status or in the occurrence of a disease, chronic condition, or type of injury that conforms to a regular seasonal pattern.

• secondary attack rate see  attack rate, secondary.

• secular trend see  trend, secular.

• selection bias see  bias, selection.

• semilogarithmic-scale line graph see  [[line graph, semilogarithmic-scale

• sensitivity the ability of a test, case definition, or surveillance system to identify true cases; the proportion of people with a health condition (or the proportion of outbreaks) that are identified by a screening test or case definition (or surveillance system).

• sentinel surveillance see  surveillance, sentinel.

• sex-specific mortality rate see  mortality rate, sex-specific.

• skewed a distribution that is not symmetrical.

• source (of infection) the person, animal, object, or substance from which an infectious agent is transmitted to a host.

• source case see  case, source.

• specificity the ability or a test, case definition, or surveillance system to exclude persons without the health condition of interest; the proportion of persons without a health condition that are correctly identified as such by a screening test, case definition, or surveillance system.

• spectrum of illness the range of manifestations a disease process can take (e.g., from asymptomatic to mild clinical illness to severe illness and death).

• sporadic an event that occurs infrequently and irregularly.

• spot map a visual display of the geographic pattern of a health problem, in which a marker is placed on a map to indicate where each affected person lives, works, or might have been exposed.

• standard deviation a statistical summary of how dispersed the values of a variable are around its mean, calculated as the square root of the variance.

• standard error (of the mean) the standard deviation of a theoretical distribution of sample means of a variable around the true population mean of that variable. Standard error is computed as the standard deviation of the variable divided by the square root of the sample size.

• statistical inference generalizations developed from sample data, usually with calculated degrees of uncertainty.

• statistical significance the measure of how likely it is that a set of study results could have occurred by chance alone. Statistical significance is based on an estimate of the probability of the observed or a greater degree of association between independent and dependent variables occurring under the null hypothesis (see also P value).

• study, analytic a study, usually observational, in which groups are compared to identify and quantify associations, test hypotheses, and identify causes. Two common types are cohort studies and case-control studies.

• study, case-control an observational analytic study that enrolls one group of persons with a certain disease, chronic condition, or type of injury (case-patients) and a group of persons without the health problem (control subjects) and compares differences in exposures, behaviors, and other characteristics to identify and quantify associations, test hypotheses, and identify causes.

• study, cohort an observational analytic study in which enrollment is based on status of exposure to a certain factor or membership in a certain group. Populations are followed, and disease, death, or other health-related outcomes are documented and compared. Cohort studies can be either prospective or retrospective.

• study, cross-sectional a study in which a sample of persons from a population are enrolled and their exposures and health outcomes are measured simultaneously; a survey.

• study, experimental a study in which the investigator specifies the type of exposure for each person (clinical trial) or community (community trial) then follows the persons’ or communities’ health status to determine the effects of the exposure.

• study, observational a study in which the investigator observes rather than influences exposure and disease among participants. Case-control and cohort studies are observational studies (see also study, experimental).

• study, prospective an analytic study in which participants are enrolled before the health outcome of interest has occurred.

• study, retrospective an analytic study in which participants are enrolled after the health outcome of interest has occurred. Case-control studies are inherently retrospective.

• subclinical without apparent symptoms.

• surveillance, active public health surveillance in which the health agency solicits reports.

• surveillance, medical monitoring of a person who might have been exposed to an infectious, chemical, radiologic, or other potentially causal agent, for the purpose of detecting early symptoms.

• surveillance, passive public health surveillance in which data are sent to the health agency without prompting.

• surveillance, sentinel a surveillance system that uses a prearranged sample of sources (e.g., physicians, hospitals, or clinics) who have agreed to report all cases of one or more notifiable diseases.

• surveillance, syndromic (1) the monitoring of the frequency of illnesses with a specified set of clinical features among a given population without regard to the specific diagnoses, if any, that are assigned to them by clinicians. (2) A system for early detection of outbreaks whereby health department staff, assisted by automated acquisition of data routinely collected for other purposes and computer generation of statistical signals, monitor disease indicators, particularly those associated with possible terrorism-related biologic and chemical agents, continually or at least daily to detect outbreaks earlier than would otherwise be possible with traditional public health methods.

• survey a systematic canvassing of persons to collect information, often from a representative sample of the population.

• survival curve a line graph that begins with 100% of the study population and displays the percentage of the population still surviving at successive points in time. A survival curve can also be used to depict freedom from a health problem, complication, or another endpoint.

• symmetrical a type of distribution where the shapes to the right and left of the central location are the same. Normal, bell-shaped distributions are symmetrical; the mean, median, and mode are the same.

• symptom any indication of disease noticed or felt by a patient.

• syndrome a combination of symptoms characteristic of a disease or health condition; sometimes refers to a health condition without a clear cause (e.g., chronic fatigue syndrome).

• syndromic surveillance see  surveillance, syndromic.

• table an arrangement of data in rows and columns. In epidemiology, the data are usually summaries of the frequency of occurrence of an event or characteristic occurring among different groups.

• table shell a table that is completely drawn and labeled but contains no data.

• table, two-by-two a two-variable table with cross-tabulated data, in which each variable has only two categories. Usually, one variable represents a health outcome, and one represents an exposure or personal characteristic.

• transmission (of infection) any mode or mechanism by which an infectious agent is spread to a susceptible host.

• transmission, airborne transfer of an agent suspended in the air, considered a type of indirect transmission.

• transmission, biologic indirect transmission by a vector in which the infectious agent undergoes biologic changes inside the vector as part of its life cycle before it is transmitted to the host (see also transmission, mechanical).

• transmission, direct immediate transfer of an agent from a reservoir to a host by direct contact or droplet spread.

• transmission, indirect transfer of an agent from a reservoir to a host either by being suspended in air particles (airborne), carried by an inanimate objects (vehicleborne), or carried by an animate intermediary (vectorborne).

• transmission, mechanical indirect transmission by a vector in which the infectious agent does not undergo physiologic changes inside the vector (see also transmission, biologic).

• transmission, vectorborne transmission of an agent by a living intermediary (e.g., tick, mosquito, or flea); considered a type of indirect transmission.

• transmission, vehicleborne transmission of an agent by an inanimate object; considered a type of indirect transmission; includes foodborne and waterborne transmission.

• trend movement or change in frequency over time, usually upwards or downwards.

• trend, secular changes occurring over a substantial period, generally years or decades.

• trial, clinical an experimental study that uses data from individual persons. The investigator specifies the type of exposure for each study participant and then follows each person’s health status to determine the effects of the exposure.

• trial, community an experimental study that uses data from communities. The investigator specifies the type of exposure for each community and then follows the communities’ health status to determine the effects of the exposure.

• trial, randomized clinical a clinical trial in which persons are randomly assigned to exposure or treatment groups.

• two-by-two table see  table, two-by-two.

• variable the degree to which a measurement, questionnaire, test, or study or any other data-collection tool measures what it is intended to measure.

• variable any characteristic or attribute that can be measured and can have different values.

• variable (or * data), * discrete a variable that is limited to a finite number of values; data for such a variable.

• variable, continuous a variable that has the potential for having an infinite number of values along a continuum (e.g., height and weight).

• variable, dependent in a statistical analysis, a variable whose values are a function of one or more other variables.

• variable, independent an exposure, risk factor, or other characteristic being observed or measured that is hypothesized to influence an event or manifestation (the dependent variable).

• variance a measure of the spread in a set of observations, calculated as the sum of the squares of deviations from the mean, divided by the number of observations minus 1 (see also standard deviation).

• vector a living intermediary that carries an agent from a reservoir to a susceptible host (see also transmission, biologic and transmission, mechanical) (e.g., mosquitoes, fleas, or ticks).

• vehicle an inanimate object that can carry an agent from a reservoir to a susceptible host (e.g., food, water, blood products, and bedding) (see also transmission, indirect).

• virulence the ability of an infectious agent to cause severe disease, measured as the proportion of persons with the disease who become severely ill or die.

• vital statistics systematically tabulated data about recorded births, marriages, divorces, and deaths.

• x-axis the horizontal axis of a rectangular graph, usually displaying the independent variable (e.g., time).

• y-axis the vertical axis of a rectangular graph, usually displaying the dependent variable (e.g., frequency — number, proportion, or rate).

• years of potential life lost (YPLL) a measure of the impact of premature death on a population, calculated as the sum of the differences between a predetermined minimally acceptable age (e.g., 65 years or current life expectancy) and the age at death for everyone who died earlier than that age.

• zoonosis an infectious disease that is transmissible from animals to humans.

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