Selection Bias

Selection bias occurs when there are systematic differences between the comparison groups in a study. For example, differences in clinical signs between the groups might lead to different disease progression or response to treatment between groups, rather than the intervention itself. Proper randomisation and/or blinding have not been achieved, which can affect the statistical analysis and internal validity of the study.

Selection bias is sometimes used to refer to an error in the selection of studies for reviews. Publication bias is a type of selection bias. Confusingly, selection bias is also sometimes used to refer to systematic differences between the study group and the general population. This leads to problems with external validity.

Glossary: Quasi-randomised trial
Glossary: Allocation concealment
Statistics in clinical trials: Bias
There are several kinds of bias that may occur in clinical trials, including selection, measurement, and publication bias. Statistics are used in clinical trials in order to avoid or correct these biases.
Critical reading of clinical study results
When performing a critical reading of clinical study results, the reader should take relevant information into account from the best available sources and should consider questions about the reliability, methodology, results, discussion, significance, and conclusions of the study.

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