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From | "Lachenbruch, Peter" <Peter.Lachenbruch@oregonstate.edu> |
To | "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu> |
Subject | st: RE: Re: pwcompare |
Date | Sat, 26 Nov 2011 10:44:03 -0800 |
I didn't see an answer to this. I suspect it is related to the fact that the standard errors in the xtmixed model account for the random effects, and so are larger. Note that the contrasts are the same in both as they are the sample means. Tonyh ________________________________________ From: owner-statalist@hsphsun2.harvard.edu [owner-statalist@hsphsun2.harvard.edu] On Behalf Of Janet Hill [janethill73@yahoo.co.uk] Sent: Friday, November 25, 2011 8:35 AM To: statalist@hsphsun2.harvard.edu Subject: st: Re: pwcompare Why does pwcompare give different results after anova and xtmixed? Using the following syntax: xtmixed lhist i.group##i.time || dog:, reml cov(id) or anova lhist group / dog|group time time#group, repeated(time) followed by pwcompare group, emptycells(reweigh) eff asbalanced post I get the following for xtmixed ------------------------------------------------------------------------------ | Unadjusted Unadjusted | Contrast Std. Err. z P>|z| [95% Conf. Interval] -------------+---------------------------------------------------------------- lhist | group | 2 vs 1 | -.4591643 .5681373 -0.81 0.419 -1.572693 .6543643 3 vs 1 | 1.200595 .525993 2.28 0.022 .1696677 2.231522 4 vs 1 | -.3857097 .525993 -0.73 0.463 -1.416637 .6452175 3 vs 2 | 1.659759 .5681373 2.92 0.003 .5462306 2.773288 4 vs 2 | .0734546 .5681373 0.13 0.897 -1.040074 1.186983 4 vs 3 | -1.586305 .525993 -3.02 0.003 -2.617232 -.5553775 ------------------------------------------------------------------------------ and for the anova ------------------------------------------------------------------------------ | Unadjusted Unadjusted | Contrast Std. Err. t P>|t| [95% Conf. Interval] -------------+---------------------------------------------------------------- group | 2 vs 1 | -.4591643 .1047385 -4.38 0.000 -.6722564 -.2460723 3 vs 1 | 1.200595 .096969 12.38 0.000 1.00331 1.39788 4 vs 1 | -.3857097 .096969 -3.98 0.000 -.5829947 -.1884248 3 vs 2 | 1.659759 .1047385 15.85 0.000 1.446667 1.872851 4 vs 2 | .0734546 .1047385 0.70 0.488 -.1396375 .2865467 4 vs 3 | -1.586305 .096969 -16.36 0.000 -1.78359 -1.38902 ------------------------------------------------------------------------------ I can see that the standard errors are different, but the question arises should I use xtmixed or anova for repeated measures? I am using Stata 12.0, update 10 Nov 2011. Thanks, Janet * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/