Re: exact versus asymptotic p-value under normal circumstances

Posted by Bruce Weaver on
URL: http://spssx-discussion.165.s1.nabble.com/Revision-control-for-SPSS-source-code-tp4267835p4269819.html

Hi Bridgette.  A couple things.

1. Can you post your table of observed counts?

2. For tables larger than 2x2, the chi-square approximation is pretty good as long as no more than 20% of expected counts are < 5, and none are < 1.  

The reference I've always used for point 2 is Yates, Moore & McCabe (1999, p. 734).  But I just noticed that Agresti says the same thing (in the book Rich U referred to earlier), and he cites Cochran (1954).

Cheers,
Bruce

Bridgette Portman wrote

One more question...

I have the SPSS Exact Tests module, which allows one to compute an exact
p-value for a chi-square test on any size table (not just 2x2). I was
taught to use exact tests under certain circumstances, e.g. when the
expected value in any cell falls below 5. My dataset is about 250, so
usually that condition doesn't apply. However, I am still finding that the
exact p-value differs significantly from the asymptotic p
value...sometimes to the extent that they lead to different conclusions.
I'm not sure whether I should rely on the exact p-value anytime it differs
from the asymptotic one, or if I should stick with the asymptotic when the
usual assumptions for chi-square are met.

For example:  On the crosstabs I just ran, X2=4.49, 3 degrees of freedom,
asymptotic sig is showing up as .212, and exact sig as .028. All expected
cell counts were greater than 5.

If I didn't have the exact tests module, I would have just gone with the
.212 and retained the null hypothesis. Now that I have exact, should I use
it or not?

Thanks for any advice,
Bridgette

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