Abstract This chapter gives understanding of measuring evidence about consequences of screening programmes, at a level needed by public health practitioners interpreting evidence for setting screening policy, or for ensuring high quality programme delivery. It takes a practical approach, illustrating with real-life case histories and examples, ranging from infant neuroblastoma to abdominal aortic aneurysm screening. The issues covered are Three Main Biases (healthy screenee effect, length time effect including overdiagnosis, lead time effect); Three Main Evaluation Methods (randomised control trials, time trend analyses, case control studies, and mention of rare conditions); Test Performance (sensitivity and specificity, positive and negative predictive value, receiver operator characteristic curves); Two Additional Sources of Information (pilot or demonstration projects, modelling); Summarising Information on all Outcomes (the numbers in the flow diagram, and decision aids) and finishing with A Note on Sound Science. Careful use of terminology is emphasised, and pitfalls with use of jargon terms (true and false, positive and negative) and misleading labels (early, late, pre-symptomatic, carcinoma in situ) are explained. The issue of overdiagnosis of inconsequential conditions is explained.
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