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

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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