FISHER is designed for testing independence for a 2×2 table.In other words, it answers the following question: Given two-way tables with the same fixed margin totals as the observed one, what is the chance of obtaining the observed cell frequencies a, b, c, and d and all cell frequencies that represent a greater deviation from expectation (see Sokal and Rohlf 1995).
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Zhang J., H. F. Rosenberg, and M. Nei (1998)
Positive Darwinian selection after gene duplication in primate ribonuclease genes.
Proc. Natl. Acad. Sci. USA 95:3708-3713.