ascertainment bias

Ascertainment bias arises when data for a study or an analysis are collected (or surveyed, screened, or recorded) such that some members of the target population are less likely to be included in the final results than others. The resulting study sample becomes biased, as it is systematically different from the target population. Ascertainment bias is related to sampling bias, selection bias, detection bias, and observer bias.

Ascertainment bias can happen when there is more intense surveillance or screening for outcomes among exposed individuals than among unexposed individuals, or differential recording of outcomes.

Ascertainment bias can occur in screening, where take-up can be influenced by factors such as cultural differences. It can occur in case-control studies in the initial identification of cases and controls, which can be skewed by relevant exposures, leading to biased estimates of associations.

In a clinical trial, if allocation concealment and blinding are lacking, then outcome ascertainment can be influenced by knowledge of the allocation.