Choosing a sensible contrast makes “prevalence bias” irrelevant in screening colonoscopy trials

Screening colonoscopy has been shown to reduce colorectal cancer incidence. However, the magnitude of this effect is debated. There is concern that some trial participants already had colorectal cancer at baseline. The screening procedure could not prevent disease occurrence in these participants, leading to “prevalence bias”. Some authors have argued that the effect of interest is confined to participants without disease at baseline, and failing to exclude prevalent cases supposedly leads to effect underestimation. Yet, the issue is debated, with other authors arguing that conventional randomized trials provide the effects that are most relevant to public health. Here we present new, formal arguments that clarify misconceptions in this debate. We show that, under mild assumptions, the so-called “prevalence bias” is not a concern when researchers are interested in estimating risk differences, rather than risk ratios. This is because of a statistical property of the causal risk difference when outcomes are rare, called “doomed-selection stability”.