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st: RE: interaction between two continuous variables


From   "Anderson, Bradley J" <BAnderson1@lifespan.org>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: interaction between two continuous variables
Date   Wed, 20 Sep 2006 13:34:23 -0400

The interaction means the the slope of one variable (say X1) varies as a
linear function of the second variable (say X2).  Something you might do is
to estimate the effect of X1 at selected values of X2 (e.g., at -2SD, at
-1SD, at the mean, at +1SD, at +2SD).  The selected values of X2 may be
guided by theory.  You might also plot the slope of X1 at selected values of
X2.  Of course, the interaction also implies that the effect of X2 varies as
a linear function of X1.  But in general, substantive considerations will
serve as a guide us here.  For example, we might be interested in describing
how the effect of depression on drug use is conditioned on age.  But we
probably wouldn't be interested in how the effect of age on drug use is
conditioned by depression.

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu]On Behalf Of stata_user
stata_user
Sent: Wednesday, September 20, 2006 11:59 AM
To: statalist@hsphsun2.harvard.edu
Subject: st: interaction between two continuous variables


Hi,

I have the following model y continuous , x1 x2 continuous

I do a regression  with

reg y cx1 cx2 cx1*cx2

with
cx1= x1 -mean (x1)
cx2= x2 -mean (x2)

both the paramters of cx1 and cx2 are signficants , which mean that the 
slope of x2 is signficant for x1=mean (x1)

the paramter of cx1*cx2 is also signficant

How to interpret the parameter of cx1*cx2 ?

My first interpretation is that  there are at least two values of x1 , where

the slopes of x2 are not parallels

but how to know, that the slope of x2 for a given value of x1=a is 
signficant?

like using a lincom command

Any comments, please

Thanks

Sami

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