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A dictionary definition of bias is 'a one-sided inclination of the mind'. It defines a systematic disposition of certain trial designs to produce results consistently better or worse than other trial designs.

Bias is an asystematic error or deviation in results or inferences. In studies of the effects of healthcare bias can arise from systematic differences in the groups that are compared (selection bias), the care that is provided, or exposure to other factors apart from the intervention of interest (performance bias), withdrawals or exclusions of people entered into the study (attrition bias) or how outcomes are assessed (detection bias). Bias does not necessarily carry an imputation of prejudice, such as the investigators' desire for particular results. This differs from conventional use of the word in which bias refers to a partisan point of view.

Bias in clinical trials has many different origins, and is dealt with in a longer essay or PDF.

Bias also occurs in diagnostic tests.