Introduction
The UK Breast Screening Programme (UKBSP) is comprised of approximately 100 individual Breast Screening Units (BSUs) that vary in size (measured by number of women screened). Previous research using UKBSP real life data (Blanks, Bennett, Wallis and Moss, 2002), attributed differences in performance, related to BSU size, to smaller units' slightly lowered cancer detection rates and positive predictive value scores (compared to large/medium units).
All BSUs on the UKBSP annually take part in the PERFORMS scheme as way of self-assessing their film-reading skills. We looked at the performance of all film-readers who had completed the last PERFORMS round (SA08) by BSU size in order to explore any group differences mediated by unit capacity.