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Re: st: Presenting Interaction in LR

From   Maarten Buis <>
Subject   Re: st: Presenting Interaction in LR
Date   Thu, 8 Dec 2011 18:57:29 +0100

On Thu, Dec 8, 2011 at 6:25 PM, Campo, Marc wrote:
> 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..

Quartiles sound fine to me, but the most important thing is that the
values make some substantive sense. There often are some values on a
scale that are especially meaningful, e.g. 12 and 16 years of
education in the American educational system (high school and college
degree). In that case I would use those values.

Hope this helps,

Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen

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