InStat and Prism give a different P value for Fisher's exact test than does another program.

Last modified January 1, 2009

The Fisher's test is called an "exact" test, so you'd think there is exactly one way to compute the P value. Not so. While everyone agrees on how to compute one-sided (one-tailed) P value, there are actually three methods to compute "exact" two-sided (two-tailed) P value from Fisher's test.

Prism and InStat (as well as QuickCalc since 5-April-2004) compute the two-sided P value using the method of summing small P values. Most statisticians seem to recommend this approach, but some programs use a different approach.

If you want to learn more, SISA provides a detailed discussion with references. Also see the section on Fisher's test in Categorical Data Analysis by Alan Agresti. It is a very confusing topic, which explains why different statististicians (and so different software companies) use different methods.

Also note two bugs that we fixed.


Keywords: Fisher

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