What screening does

This chapter gives a deeper understanding of screening, and of the diverse consequences it brings. It illustrates the overdetection problem with reference to mammography breast screening programmes, and explains the ‘popularity paradox’ that this leads to. In the early days, the simplistic notion that screening must automatically be beneficial meant that people only asked ‘why are we not doing it?’ Later scientific challenges brought a new question, ‘How do we tell if screening succeeds in reducing risk?’ This served well as a driving force for better evaluation, but it ignored the need to assess harmful consequences. Growing experience has revealed the need to ask ‘what are all the consequences?’ Different observers see some consequences more starkly than others depending on their viewpoint. This chapter explains why it is important to consider a complete overview of an entire programme and to recognise that overdetection, overtreatment and underdetection, are inevitable features even when screening is delivered to a high quality.