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Re: st: Question on interpreting interactions in linear regression models


From   Charles Koss <hqtiger@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Question on interpreting interactions in linear regression models
Date   Tue, 15 Dec 2009 09:52:57 -0600

Can you please provide the reference to the paper that you are reading?

-- 
Charles

On Tue, Dec 15, 2009 at 9:49 AM, Padmakumar Sivadasan
<pmathru2@illinois.edu> wrote:
> Hello,
>
> This is a basic question about interpreting interactions in linear
> regression models that I hope someone would be able answer or guide me
> to a useful reference.
>
> I was reading a paper in which the authors are interested in studying
> whether the effect of a continuous variable (X) on a continuous
> outcome variable (Y) varies between two periods (P=0 and P=1). They
> test this using interactions with the following linear regression
> model  Y=b1+b2X+b3P+b4XP
>
> They find that b2 is significantly different from zero, while b4 and
> (b2+b4) are not significantly different from zero. The authors
> conclude that X has an influence on Y in the first period (i.e. when
> P=0) because b2 is significant. Further, they state that the influence
> of X in the second period (i.e. when P=1) is given by (b2+b4) and this
> is not significantly different from zero, implying that X has no
> influence on Y in the second period. Thus, the effect of X on Y varies
> between the two periods.
>
> Is this interpretation of the authors correct? Does not the fact that
> the interaction coefficient (b4) is non-significant directly imply
> that there is no change in the effect of X between the two periods?
>
> Thanks
>
> Padmakumar
>
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