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T test: Manually vs. with Stata
From
Jen Zhen <[email protected]>
To
[email protected]
Subject
T test: Manually vs. with Stata
Date
Tue, 29 Mar 2011 14:42:27 +0200
Dear list members,
I have a question that makes me feel very stupid, yet is troubling me:
I would like to compute the T-statistic to test that there is no
difference in variable between groups G1 and G2.
The respective summary statistics are:
G1: Mean1=16.59, SD1=1.05, N1= 330
G2: Mean2=18.82, SD2=1.26, N2=155.
Manually I would use these to compute SE1=SD1/sqrt(N1)=0.057800598,
SE2=SD2/sqrt(N2)=0.101205635, SE_pooled=sqrt(SE1^2+SE2^2)=0.116548229,
and then T=(Mean1-Mean2)/SE_pooled=-19.13370984.
However, when I use Stata's -ttest-, I get T=20.4468. I suspect that
Stata is correct on this and that it does some degrees of freedom
correction, which I'm missing when doing this manually, but I'm not
sure where I'm going astray. Would anyone happen to know?
Thanks a lot,
JZ
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