Encyclopedia Entry

Fisher Exact Probability Test

Louis M. Hsu

The Fisher exact probability test (also called the Fisher-Irwin test) is one of several tests that can be used to detect whether one dichotomous variable is related to another. The rationale of this test, as well as its principal advantages and limitations, can be presented in the context of the following hypothetical small randomized experiment designed to determine whether a dichotomous “treatment” variable (Drug vs. Placebo) is related to a dichotomous “outcome” variable (Survival vs. Death). Drug Placebo Row Totals Survived X = 3 0 3 Died 0 3 3 Column total 3 3 6 Treatment 1 Treatment 2 Row Totals Success X = a b N s Failure c d N f N 1 N 2 N A physician believes that a new antiviral drug might be effective in the treatment of SARS (severe acute respiratory syndrome). Assume that the physician carries out a randomized double-blind drug efficacy study ...

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