14 March 2022 - Bias is a systematic, non-random error in the estimation of a treatment effect or the effect of an exposure or risk factor. Bias can lead to invalid results in observational studies and randomised clinical trials. 

Bias is often broadly categorised into 3 groups: confounding, information (or measurement) bias, and selection bias.

Selection bias is a general term describing bias that occurs when study participants are identified in a manner such that they are no longer representative of the target population. This can occur when an exposure and outcome each influence a common third variable—the collider—and that variable has been controlled for in the statistical analysis of the study data. Collider bias threatens the internal validity of a study and the accurate estimation of causal relationships.

Read JAMA Guide to Statistics and Methods