Hi Andres,
The z=0.385 and p=0.700 suggests that this is a two-sided test. A z value of 0.385 has an area of 0.350 in the upper tail of the normal distribution, therefore an area of 0.700 for a two-sided test.
Cheers, Garry
________________________________
From: [email protected] on behalf of Andres Gonzalez Rangel
Sent: Fri 03/10/08 00:13
To: [email protected]
Subject: st: Two-sided P value for Wilcoxon (Mann-Whitney) rank-sum test
Dear Sirs,
It is possible to make a two-sided test in a non-parametric command like ranksum? I know that signtest can, but it needs the format "signtest var1 = var2", and I need something like "signtest var, by(group)" which is only allowed with ranksum.
The problem is that, contrary to ttest which returns one-sided and two-sided tests of significance for alternative hypothesis, ranksum returns only one P value, which I suppose is a one-sided test of significance for the Ho.
. ranksum var, by(group)
Two-sample Wilcoxon rank-sum (Mann-Whitney) test
group | obs rank sum expected
-------------+---------------------------------
no | 39 1842 1794
yes | 52 2344 2392
-------------+---------------------------------
combined | 91 4186 4186
unadjusted variance 15548.00
adjustment for ties -1.98
----------
adjusted variance 15546.02
Ho: tnfap(group==no) = tnfap(group==yes)
z = 0.385
Prob> |z| = 0.7003
I need the Pr of Ho against Ha: diff != 0, like ttest does
Thanks in advance for your kind help.
Best Regards,
Andr�s Gonz�lez Rangel
Epidemiology Graduate Student
Colombia
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