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Am J Epidemiol. 1991 Nov 1;134(9):1003-8.

Response bias in a case-control study: analysis utilizing comparative data concerning legal abortions from two independent Swedish studies.

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Department of Cancer Epidemiology, Radiumhemmet, Karolinska Institute, Stockholm, Sweden.


Independent reports of legal abortions in two Swedish epidemiologic studies of breast cancer in young women, covering the same women and overlapping the same time period, have been compared in order to estimate a putative response bias. One study used case-control methods and obtained data by retrospective interviews from 317 cases and 512 controls. The other study was based on objectively documented information froma nationwide registry covering legally induced abortions. Analysis demonstrated a ratio between the odds ratios from the two studies of 1.5 (95 percent confidence interval 1.1-2.1) and an observed ratio of 22.4 (p less than 0.007) between underreporting of previous induced abortions among controls relative to overreporting among cases. This response bias may explain the tendency toward increased risk of breast cancer which, according to several case-control studies, appears to be associated with induced abortion.


Possible response bias in a retrospective study of induced abortion and premenopausal breast cancer in Swedish women was examined by comparing 2 epidemiologic studies on the same women whose abortions were either reported in interviews or ascertained from government registries. The retrospective case-control study covered 317 women diagnosed with breast cancer before age 45; these women were interviewed at home. The population-based study accessed a legal abortion registry kept from 1939-1974 on the same women. This analysis employed the ratio Q of the 2 odds ratios between women who had cancer and controls who had abortions. There were 26 cancer cases and 44 controls reporting abortions, but 24 cases and 59 controls with abortions in the registry. The ratio Q between these odds ratios was 1.5, indicating bias. There was no evidence of selective forgetfulness by cases. Probably because of the sensitive nature of abortion, and the seriousness of malignant disease, reporting of abortion was more accurate among breast cancer cases, suggesting underreporting by healthy controls.

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