Cumulative incidence rate

Proportion of the cumulative incidence to the total population.

Cumulative incidence curves mRNA versus Placebo

Other names

Synonyms and other names used include:

• Attack rate
• Risk
• Probability of developing disease
• Cumulative incidence

Definition

Incidence proportion is the proportion of an initially disease-free population that develops disease, becomes injured, or dies during a specified (usually limited) period of time. Synonyms include attack rate, risk, probability of getting disease, and cumulative incidence. Incidence proportion is a proportion because the persons in the numerator, those who develop disease, are all included in the denominator (the entire population).

Method for calculating incidence proportion (risk)

Number of new cases of disease or injury during specified period Size of population at start of period

Examples

Examples for Calculating Incidence Proportion (Risk)

1. Example A: In the study of diabetics, 100 of the 189 diabetic men died during the 13-year follow-up period. Calculate the risk of death for these men.

• Numerator = 100 deaths among the diabetic men
• Denominator = 189 diabetic men
• 10n = 102 = 100
• Risk = (100 ⁄ 189) × 100 = 52.9%

1. Example B: In an outbreak of gastroenteritis among attendees of a corporate picnic, 99 persons ate potato salad, 30 of whom developed gastroenteritis. Calculate the risk of illness among persons who ate potato salad.

• Numerator = 30 persons who ate potato salad and developed gastroenteritis
• Denominator = 99 persons who ate potato salad
• 10n = 102 = 100
• Risk = “Food-specific attack rate” = (30 ⁄ 99) × 100 = 0.303 × 100 = 30.3%

Properties and uses of incidence proportions

• Incidence proportion is a measure of the risk of disease or the probability of developing the disease during the specified period. As a measure of incidence, it includes only new cases of disease in the numerator.
• The denominator is the number of persons in the population at the start of the observation period. Because all of the persons with new cases of disease (numerator) are also represented in the denominator, a risk is also a proportion.

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