Selection bias may occur during identification of the study population. The ideal study
population is clearly defined, accessible, reliable, and at increased risk to develop the
outcome of interest. When a study population is identified, selection bias occurs when the
criteria used to recruit and enroll patients into separate study cohorts are inherently different.
This can be a particular problem with case-control and retrospective cohort studies where
exposure and outcome have already occurred at the time individuals are selected for study
inclusion 18. Prospective studies (particularly randomized, controlled trials) where the
outcome is unknown at time of enrollment are less prone to selection bias.