Correcting for non‐compliance bias in case–control studies to evaluate cancer screening programmes

Summary. In the evaluation of uncontrolled service screening programmes for cancer, the case–control design is sometimes used, in which people who die from the disease in question are compared with live controls with respect to screening histories. Such a design tends to yield estimates of relative mortality in voluntary participants compared with non‐participants. This may bias results, since compliers and non‐compliers may differ a priori in ways which are not related to screening but which nevertheless affect the risk of death from the disease. We present a simple method, employing external data from previously published randomized controlled trials of screening, of correction for this bias. We illustrate it by using data from a case–control study performed within the invited arm of the Malmö mammographic screening trial, a prospective study from the service screening programme in two counties in Sweden, and a matched case–control study of mammographic screening in Florence, Italy.