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PSPP-BUG: [bug #26936] T-TEST reports 2*p instead of p
From: |
Mike Griffiths |
Subject: |
PSPP-BUG: [bug #26936] T-TEST reports 2*p instead of p |
Date: |
Tue, 14 Jul 2009 19:06:11 +0000 |
User-agent: |
Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; GTB6; .NET CLR 2.0.50727; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729) |
Follow-up Comment #2, bug #26936 (project pspp):
I'm afraid the answer given is incorrect. Furthermore, I think this is a
serious error in PSPP.
Unfortunately the person who originally logged my item incorrectly summarised
it in the title. The p-value calculated by PSPP is not 2*p, but 2-p. This is
nothing to do with the difference between one and two tailed values. If you
correctly calculate the p-value from the inverse CDF you can never come up
with a p-value greater than 1, whether 1 or 2 tailed. (The circumstances when
you are likely to calculate a p-value greater than 1 are when you are doing
corrections for multiple comparisons, and certainly the value then should be
capped at 1.00. Probability can never be more than 100%.)
To reiterate what I originally reported. My test data were: variable A = 11,
1, 1. Variable B = 2, 1, 1. The correct p-value for a paired-samples t-test
(from SPSS) is .423 2-tailed, hence .212 1-tailed. The p-value reported by
PSPP is 1.58 2-tailed.
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