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Despite the widely held concern that reporting bias may distort the relation between a history of induced abortion and risk of breast cancer in interview-based case-control studies, direct evidence is scarce (1-3). More insight into this matter is essential for weighing the results of the many case-control studies (n = 26 (4-7)), all based on interviews or questionnaires, and the few follow-up studies (n = 3 (8-10)) that examined breast cancer risk following induced abortion. Thus, the present attempt of Tang et al. (11) to quantify reporting bias in a sample of participants of two such case-control studies is of great interest. It is known that induced abortions are markedly underreported in the general population, most probably because of the sensitive nature of the procedure (12, 13). The question now is whether women with breast cancer might be more willing to report an abortion history than controls, for instance, because they are more motivated for a study directed at understanding their illness. Less underreporting by cases than by controls would spuriously increase the odds ratios in interview-based case-control studies. So far, most information on reporting bias has been based on indirect evidence. Newcombe et al. (14) found a more strongly increased risk of breast cancer for abortions that took place before the date of legalization rather than after that date. Palmer et al. (7) reported a higher odds ratio for women who were interviewed during an earlier time period and thus were more likely to have had their abortions before or shortly after abortion became legal. However, Daling et al. (5) did not find effect modification by legalization date. Furthermore, the legalization date might be associated with true effect modifiers of the association between induced abortion and breast cancer risk, such as age at the induced abortion, length of gesta-

Invited Commentary: Reporting Bias in Case-Control Studies on Induced Abortion and Breast Cancer