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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
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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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