Case-control evaluation of breast cancer screening efficacy.

The feasibility and validity of using case-control methods to evaluate the efficacy of breast cancer screening were investigated using data from the Health Insurance Plan of Greater New York Breast Cancer Screening Trial. Women who died of breast cancer were compared with individually matched controls of the same age who had equal or greater survival times. To minimize the effects of self-selection bias, most analyses focused on the 95 cases and 380 matched controls who had been screened at least once and for whom covariate data were available. A statistically significant effect for being screened more than once versus being screened once, uncorrected for healthy-screenee bias, was found (odds ratio = 0.13, p less than 0.001). After correction for healthy-screenee bias, the effect was not statistically significant (odds ratio = 0.54, p = 0.15). Possible explanations for this statistical nonsignificance are: 1) correction for healthy-screenee bias entails loss of power and 2) the correction procedure used may, under some circumstances, result in bias toward the null hypothesis. Further research into means of eliminating healthy-screenee bias is needed to improve the applicability of case-control methods to the evaluation of screening efficacy.