Tuesday, February 28, 2006

st: fishers exact test


I have a 2x2 table:

first row: 41 12 second row: 6 1

Running tabi gives a p-value for Fisher's exact test of 1.0 (two sided) and 0.52 (one sided). The hypothesis suggests using two sided value.

A reviewer commented that p-values, unless the data is quite unique, should not be 1.0

Why is the p-value for Fisher's exact test exactly 1.0? Does this make sense? Moreover, after viewing "return list," the p-value isn't 1.0 but 1.00000000012. How can a p-value exceed 1.0?

Also, looking on the web, some sites calculate the following:

Two sided p-values for p(O>=E|O<=E) p-value= 1.0000000000* (the sum of small p's) Two sided p-value for p(O>E|O<E) p-value= 0.6385656450 (the sum of small p's)

Should I be using a different test?

Thank you, Richard Lenhardt

* * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/


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