Posted by
Anthony Babinec on
URL: http://spssx-discussion.165.s1.nabble.com/Revision-control-for-SPSS-source-code-tp4267835p4268308.html
The exact test always "trumps" the asymptotic test; you should use the exact
test
result. The rule of thumb about the expected value in any cell falling below
5 is just
that - a rule of thumb - and no more. Exact tests are useful in situations
of sparseness,
skewness, extreme imbalance, and small sample sizes. It is difficult to
state before
the fact when you would get a different result.
Tony Babinec
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-----Original Message-----
From: SPSSX(r) Discussion [mailto:
[hidden email]] On Behalf Of
Bridgette Portman
Sent: Monday, March 28, 2011 3:34 PM
To:
[hidden email]
Subject: exact versus asymptotic p-value under normal circumstances
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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