Selection Bias

Selection Bias is a type of bias that occurs when the sample used in a study is not representative of the population it's meant to represent. This can lead to inaccurate results and conclusions.

Definition

Selection bias refers to the practice of selecting individuals, groups or data for analysis in such a way that proper randomization is not achieved, thereby ensuring that the sample obtained is not representative of the population intended to be analyzed. It is sometimes referred to as the selection effect. The phrase "selection bias" most often refers to the distortion of a statistical analysis, resulting from the method of collecting samples.

Types of Selection Bias

There are several types of selection bias, including:

• Sampling bias: This occurs when the sample used in a study is not representative of the population it's meant to represent. This can lead to inaccurate results and conclusions.

• Time interval: A trial may be terminated early at an extreme value (often for ethical reasons), but the extreme value is likely to be reached by the variable with the largest variance, even if all variables have a similar mean.

• Data: When specific subsets of data are chosen to support a conclusion or rejection of bad data on arbitrary grounds, instead of according to previously stated or generally agreed criteria.

• Attrition: Attrition bias is a kind of selection bias caused by attrition (loss of participants) discounting trial subjects/tests that did not run to completion.

Effects of Selection Bias

Selection bias can have a number of effects on a study, including:

• Validity: Selection bias can affect the validity of a study, making the results less applicable to the wider population.

• Reliability: If the selection bias is consistent, it can affect the reliability of the study, making the results less repeatable.

• Generalizability: Selection bias can affect the generalizability of a study, making the results less applicable to other settings or groups.

Prevention and Control

There are several ways to prevent or control selection bias, including:

• Randomization: This is the practice of assigning subjects to different groups randomly, so that each subject has an equal chance of being in any group.

• Stratification: This involves dividing subjects into subgroups based on certain characteristics, and then selecting samples from each subgroup.

• Matching: This involves pairing subjects who have similar characteristics, and then comparing the outcomes of the pairs.

See Also

• Bias (statistics)
• Confounding
• Observational error
• Sampling (statistics)

References

Selection Bias Resources
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