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Biostatistics cheat sheets

One-page reference sheets for the biostatistics a medical exam asks about: strict definitions first, then the formulas written several ways, where the numbers actually come from, and the errors that survive into practice. Read each one as a searchable web page, or download it as a printable PDF.

Predictive values: PPV and NPV

Sensitivity · specificity · prevalence · likelihood ratios · Bayes

Why a 99%-accurate test can be wrong most of the time, and how to compute the answer. Strict definitions of sensitivity, specificity, prevalence and the predictive values; the 2×2 table; Bayes' theorem in four equivalent forms; where the inputs come from; typical values for real tests; and the classic traps, including the inverse fallacy and the case–control table.

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PPV and NPV cheat sheet, page 1: definitions of sensitivity, specificity, prevalence and the predictive values, the 2x2 contingency table, and Bayes' theorem written four ways. PPV and NPV cheat sheet, page 2: where sensitivity, specificity and prevalence come from, typical values for real diagnostic tests, and the common errors in interpreting them.
The two-page PPV / NPV reference card. Page one: definitions, the 2×2 table and Bayes. Page two: where the inputs come from, typical values and the classic errors.