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st: Presenting Interaction in LR


From   "Campo, Marc" <MCampo@mercy.edu>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Presenting Interaction in LR
Date   Thu, 8 Dec 2011 17:25:14 +0000

Dear Statalisters:

We are running a Linear Regression model where we are interested in the effect of a dichotomous predictor (GROUP) on a ratio level rehabilitation outcome scale.   It is a pre/post situation where we have adjusted for baseline values (BASELINE) of the scale.   There is an interaction between GROUP and BASELINE.   People at the low end of the scale do better in one group and at the higher end do better in the other (but not much of a difference in clinical terms).   

I was going to present the coefficients for GROUP at varying values of baseline.  BASELINE however, is somewhat skewed and so when I calculate the coefficient for group at the mean and 1SD above and below the mean BASELINE value we end up with a lower coefficient that is fairly close to the minimum value.   Are there other choices?   25, 50, 75Th percentiles?   Graphs instead?   I don't think transforming BASELINE adds enough to be worth the complexity and the relationship is fairly linear as is..

Thanks so much for any ideas!


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