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Selection bias affects inclusion of individuals in the study and results in the study sample not being representative of the source population. This may occur in the initial selection process, or may be a result of nonresponse or losses to follow-up during the study period. The mechanisms by which selection bias can enter a study will depend on the [[study design]]:
Selection bias affects inclusion of individuals in the study and results in the study sample not being representative of the source population. This may occur in the initial selection process, or may be a result of nonresponse or losses to follow-up during the study period. The mechanisms by which selection bias can enter a study will depend on the [[study design]]:
===Descriptive or cross sectional studies===
===Descriptive studies===
In these studies, lack of compliance ('non-response') may result in selection bias, as participation amongst selected individuals is very rarely 100%. It is for this reason that compliance should be maximised as much as possible, through the use of reminders or incentives to participate.
In these studies, lack of compliance ('non-response') may result in selection bias, as participation amongst selected individuals is very rarely 100%. It is for this reason that compliance should be maximised as much as possible, through the use of reminders or incentives to participate.
===Cross sectional studies===
Although non-response is also the main route of selection bias entering these studies, it should be noted here that analytic studies differ from descriptive studies in how bias can occur, due to the aim of the study. In these cases, both exposure and outcome should be considered - if selection is associated with both of these (meaning that selection of 'diseased' individuals is affected by their exposure category), then bias will result. If selection is 'only' associated with 'either' exposure status or outcome, then bias will not result.
===Case-control studies===
===Case-control studies===