title: The unprejudiced basis of bias.
abstract: The results of a study can be invalid if the study population is not chosen properly. Four hypothetical examples show how unintentional selection bias can occur and how to recognize it in both prospectiveand retrospective studies.
mesh_terms: D002986:Clinical Trials as Topic; D006801:Humans; D011446:Prospective Studies; D012107:Research Design; D012189:Retrospective Studies