Screening is a means of detecting a disease in its pre-symptomatic stage (that is, when no symptoms are apparent). The number of people screened is typically large and those showing evidence of disease will then normally receive further (typically more costly and/or more risky) confirmatory tests and treatment. The theory is that early detection enables early treatment, though whether this is more effective (than, say, watchful waiting) or more cost-effective, is an empirical matter rather than one of principle and ought to be so treated. Screening programmes are subject to problems with false positives and false negatives. See also Sensitivity and Specificity. Screening programmes are also subject to bias (a common one is lead-time bias). A screening programme is a health technology that is often a suitable subject for a clinical trial and for cost-effectiveness or cost-utility analysis.

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