Fig. 1

Hypothesis of this study: Self-selection bias can be isolated from other forms of bias by comparing participants with interval breast cancers with non-participants with breast cancer regarding death from all causes other than breast cancer. As shown by bold and normal print, the extent and even direction of the assumed lead and length time biases can differ across the different comparisons. Lead Time Bias: Lead time of unlimited length (maximum is the study period) can be expected in the comparison of survival across MSP participants with screen-detected tumours and non-participants. In the comparison of MSP participants with screen-detected breast cancer and MSP participants with interval breast cancers, the lead time can only reach a maximum of 2.5 years, equivalent to the time interval that defines an interval cancer. In the same two comparisons, a sort of “lead time” is also expected when considering death from all causes other than breast cancer, due to the advanced start of the observation period in MSP participants with screen-detected breast cancers. Length Time Bias: Length time bias might also be present in all two-way comparisons. However, the effect and even direction cannot be well predicted, as the tumours in patients with interval cancers most probably reflect a highly heterogeneous sample regarding growth rate and prognostic factors [24]. For death from all causes other than breast cancer, however, there is no length time bias. MSP Mammography screening programme