The case-control study is an efficient study design for evaluating the effect of cancer screening. This study method enables calculation of the percentage by which the risk of mortality from cancer decreases in participants in a screening programme if the carcinoma is detected early. Reductions in mortality of 15-65% have been published for participants in a population screening programme for breast, cervical and colorectal cancer. The case-control study is an observational study, in other words it does not have an experimental design. There is, therefore, a risk that confounding and self-selection bias may cause over- or underestimation of the mortality reduction. It is, therefore, important that at publication investigators indicate the extent to which they have succeeded in minimalising the occurrence of these forms of bias in their study design and data analysis.