PPV, NPV & the Prevalence Effect

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PPV = TP/(TP+FP) = if positive, chance truly diseased. NPV = TN/(TN+FN). CRITICAL: PPV/NPV depend on prevalence (pre-test probability). Same test, low prevalence → low PPV (more false positives) → why population screening throws up false alarms. ScreeningPrevalenceDemo: fix sens/spec, slide prevalence, watch PPV collapse. Worked example at 1% vs 50% prevalence. Highest-yield exam concept.

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