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st: RE: Interaction


From   "Scott Merryman" <smerryman@kc.rr.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Interaction
Date   Thu, 6 Oct 2005 17:30:42 -0500

Perhaps I am confused, but if (in equation 1) b2 = .29 than would not a
one-unit change in group be a 29% increase in y?

Regarding Eq. 2: The marginal effect of group on y is not simply b2 but:

 b2 + b3*time

and the variance would be:

 var[b2] + time^2*var[b3] + 2*time*cov[b2,b3], 

where time is at some given value; so whether or not the variable group is
statistically significant cannot be determined solely by the looking at the
coefficient and standard error on group (b2).


Scott



> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-
> statalist@hsphsun2.harvard.edu] On Behalf Of lm335@drexel.edu
> Sent: Thursday, October 06, 2005 11:28 AM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: Interaction
> 
> Dear Statalisters,
> 
> My question is in relation to my previous query about interaction terms. I
> have two regression models:
> 1) ln(y)= constant + b1*(time) + b2*(group)
> 
>  In this model group is an indicator variable denoting membership in
> group1 or group2. On fiting this model, the value of b2 was .29 indicating
> a 34 percent additional increase in y for the people in group2.
> 
> The second model that I fitted included the interaction between group and
> time.
> 
> 2) ln(y)= constant + b1*(time) + b2*(group) + b3*(group*time)
> 
> In this model , the group is not significant but the interaction effect
> is. But the effect of the interaction reduces to 2.6 percent (b1=.0822,
> b3=.026).
> 
> How does one explain (In plain English)  the drastic reduction in the
> effect of time on y for the group2 people between the two models?
> 
> 
> Thank you in advance for any suggestions
> 
> Leny Mathew



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