Use when you must test a 2x2 contingency table with small expected counts: run the exact Fisher test under the fixed-margin null, compute the hypergeometric probability of the observed table, enumerate every table with the same margins, sum the one-tailed and two-tailed exact p-values, compute the odds ratio with the Haldane-Anscombe zero-cell correction, and return the small-expected-count verdict that recommends the exact test over the large-sample approximation.
