In epidemiological research, recall bias is a systematic error caused by differences in the accuracy or completeness of the recollections retrieved ("recalled") by study participants regarding events or experiences from the past. Sometimes also referred to as response bias, responder bias or reporting bias, this type of measurement bias can be a methodological issue in research that involves interviews or questionnaires (potentially leading to differential misclassification of various types of exposure). Recall bias can be a particular concern in retrospective studies that use a case-control design to investigate the etiological causes of a disease or psychiatric condition. For example, in studies of risk factors for breast cancer, women who have had the disease may search their memories more thoroughly than unaffected controls to try to recall exposure to factors that have been mentioned in the press, such as use of oral contraceptives.
Relatedly but distinctly, the term might also be used to describe an instance where a survey respondent intentionally responds incorrectly to a question about their personal history which results in response bias. As a hypothetical example, suppose that a researcher conducts a survey among women of group A, asking whether they have had an abortion, and the same survey among women of group B. If the results are different between the two groups, it might be that women of one group are less likely to have had an abortion, or it might simply be that women of one group who have had abortions are less likely to admit to it. If the latter is the case, then this would skew the survey results; this is a kind of response bias. (It is also possible that both are the case: women of one group are less likely to have had abortions, and women of one group who have had abortions are less likely to admit to it. This would still affect the survey statistics.)
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