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Am J Epidemiol. 1988 Feb;127(2):205-12.

Potential for bias in case-control studies of oral contraceptives and breast cancer.

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Department of Preventive and Social Medicine, University of Otago, Dunedin, New Zealand.



The discrepancies between the findings of the 6 large case-control studies to study the association between oral contraceptive (OC) use and breast cancer diagnosed in the 1980s may be due to chance or bias. The likelihood that chance played a role is suggested by the large numbers of subgroups examined in each study, the inconsistencies in the findings of different studies, and the wide confidence intervals around most of the relative risks. The most serious potential problem in case-control studies is that the procedures used to select cases and controls may produce groups that are not truly comparable. Recall bias is likely to contaminate information about the duration and type of part OC use. In addition, the more frequent examination of the breasts of women using OCs can produce surveillance bias. 6 procedures are recommended to minimize bias in future case-control studies of OC use and breast cancer: 1) whenever possible, cases and controls should be selected from an entire community; 2) if hospital controls need to be used, there should be explicit criteria for selecting them and the proportions of OC users in each diagnostic group should be presented; 3) women interviewed should not be aware of the study's hypotheses; 4) interviewers should be kept blind as to whether a subject is a case or control; 5) the possibility of recall bias should be investigated by comparing contraceptive histories from a sample of cases and controls with an independent source of information (preferably recorded before cancers were diagnosed); and 7) the interview should include questions about the frequency of breast examinations, so that any effects of more frequent surveillance of OC users can be controlled for.

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