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st: Re;pwcompare, repeated measures and xtmixed
From
Arti Rayit <[email protected]>
To
"[email protected]" <[email protected]>
Subject
st: Re;pwcompare, repeated measures and xtmixed
Date
Sun, 25 Sep 2011 19:50:35 +0100 (BST)
Hello,
I am trying to get to grips with pwcompare and it's use with repeated measures with anova and xtmixed in Stata 12.1. On a simple repeated measures design I get the following:
anova f i.j pat,rep(j)
. pwcompare j,eff
Pairwise comparisons of marginal linear predictions
Margins : asbalanced
------------------------------------------------------------------------------
| Unadjusted Unadjusted
| Contrast Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
j |
2 vs 1 | -3.875 1.351256 -2.87 0.009 -6.685091 -1.064909
3 vs 1 | -4 1.351256 -2.96 0.007 -6.810091 -1.189909
4 vs 1 | -2.375 1.351256 -1.76 0.093 -5.185091 .4350907
3 vs 2 | -.125 1.351256 -0.09 0.927 -2.935091 2.685091
4 vs 2 | 1.5 1.351256 1.11 0.280 -1.310091 4.310091
4 vs 3 | 1.625 1.351256 1.20 0.243 -1.185091 4.435091
------------------------------------------------------------------------------
Yet when I use the following:
xtmixed f i.j, || pat:,cov(id)
. pwcompare j,eff
Pairwise comparisons of marginal linear predictions
Margins : asbalanced
------------------------------------------------------------------------------
| Unadjusted Unadjusted
| Contrast Std. Err. z P>|z| [95% Conf. Interval]
-------------+----------------------------------------------------------------
f |
j |
2 vs 1 | -3.875 1.154982 -3.36 0.001 -6.138724 -1.611276
3 vs 1 | -4 1.154982 -3.46 0.001 -6.263724 -1.736276
4 vs 1 | -2.375 1.154982 -2.06 0.040 -4.638724 -.1112761
3 vs 2 | -.125 1.154982 -0.11 0.914 -2.388724 2.138724
4 vs 2 | 1.5 1.154982 1.30 0.194 -.7637239 3.763724
4 vs 3 | 1.625 1.154982 1.41 0.159 -.6387239 3.888724
------------------------------------------------------------------------------
and the probability for the 4 - 1 contrast goes from non significant to significant.
Have I misused pwcompare or is my specification for xtmixed incorrect?
Thanks.
Arti.
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