Extended Data Fig. 4: Evidence for the gatekeeper effect in retrospective datasets.

a, b, Graphs show the change in observed reader sensitivity in the UK (a) and the USA (b) as the cancer follow-up interval is extended. At short intervals, measured reader sensitivity is extremely high, owing to the fact that biopsies are only triggered based on radiological suspicion. As the time interval is extended, the task becomes more difficult and measured sensitivity declines. Part of this decline stems from the development of new cancers that were impossible to detect at the initial screening. However, steeper drops occur when the follow-up window encompasses the screening interval (36 months in the UK; 12 and 24 months in the USA). This is suggestive of what happens to reader metrics when gatekeeper bias is mitigated by another screening examination. In both graphs, the number of positives grows as the follow-up interval is extended. In the UK dataset (a), it increases from n = 259 within 3 months to n = 402 within 39 months. In the US dataset (b), it increases from n = 221 within n = 3 months to 553 within 39 months.