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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 or cross sectional studies===
In these studies, lack of compliance may result in selection bias, as participation amongst selected individuals is very rarely 100% in these cases.
===Case-control studies===
===Case-control studies===
===Cohort studies===
===Cohort studies===
Selection bias in cohort studies is generally less likely than in other studies, as selection for participation in the study generally precedes the development of the outcome of interest. However, losses to follow-up provide a mechanism by which selection bias may be introduced. For example, in the case of a perceived 'risky behaviour', it is plausible that individuals engaged in this may be less likely to remain in the study, meaning that these individuals (which may be more likely to experience the outcome) will be lost from the study.
Selection bias in cohort studies is generally less likely than in other studies, as selection for participation in the study generally precedes the development of the outcome of interest. However, losses to follow-up provide a mechanism by which selection bias may be introduced. For example, in the case of a perceived 'risky behaviour', it is plausible that individuals engaged in this may be less likely to remain in the study, meaning that these individuals (which may be more likely to experience the outcome) will be lost from the study.
==Information bias==
==Information bias==