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st: Interaction

From   <>
Subject   st: Interaction
Date   Thu, 06 Oct 2005 12:28:03 -0400

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

fn:Leny Mathew
tel;work:215 762 2069
org:Drexel University College of Medicine;OB/GYN control
title:Data Analyst

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