Healthy user bias

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The healthy user bias is a bias that can damage the validity of clinical trials testing the efficacy of particular therapies. Specifically, it is a selection bias: the kind of subjects that voluntarily enroll in a clinical trial and actually follow the experimental regimen are not representative of the general population. They can be expected to on average be healthier as they are concerned for their health and are predisposed to follow medical advice, both factors that would aid one's health. In a sense, being healthy/active about one's health is a precondition for becoming a subject of the study, an effect that can appear under other conditions such as studying particular groups of workers (ie. someone in ill-health is unlikely to have a job as manual laborer).