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‘Depletion of the susceptibles’ is a phrase occasionally used to describe a form of selection bias where harmful exposures can appear protective.1 2 This bias, which affects evidence interpretation, may be difficult for clinicians to recognise, because study participants may represent a random sample of people similar to a clinician’s patients. In this letter, we explain the bias through a story, a table and basic arithmetic to help those unfamiliar with other explanations using hazard ratios,3 inverse probability weighting4 or directed acyclic graphs conditioning on survival.5
Imagine a …
Contributors All authors conceived the concept and contributed to the final manuscript.
Competing interests None declared.
Patient consent Not required.
Provenance and peer review Not commissioned; internally peer reviewed.