Calculate the Fisher exact test probability for a 2×2 contingency table.
Fisher’s exact test computes the hypergeometric probability directly, making it valid at any sample size. Logarithms of factorials are used to avoid overflow with larger counts. Chi-square becomes unreliable when any expected count falls below five, which is exactly when Fisher’s exact test should be used instead.
Fisher Exact Test
P = (r₁! r₂! c₁! c₂!) ÷ (N! a! b! c! d!) for the hypergeometric probability of one table
P = (r₁! r₂! c₁! c₂!) ÷ (N! a! b! c! d!) for the hypergeometric probability of one table Fisher’s exact test computes the hypergeometric probability directly, making it valid at any sample size. Logarithms of factorials are used to avoid overflow with larger counts.
Chi-square becomes unreliable when any expected count falls below five, which is exactly when Fisher’s exact test should be used instead.
This calculator takes 4 inputs: Cell A, Cell B, Cell C, Cell D. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.