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Re: st: Interaction terms


From   Richard Williams <richardwilliams.ndu@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Interaction terms
Date   Tue, 03 May 2011 09:28:59 -0500

At 08:05 AM 5/3/2011, lreine ycenna wrote:
(2) when I run regress y ov edu wealth eduxwealth ovxedu ovxwealth
ovxeduxwealth, OV has a negative coefficient,

 but it changes to positive when I run regress y ov wealthxedu
ovxwealthxedu. What does this suppose to mean?

In your 2nd regression command, you are not including the main effects of wealth and education, or all of the 2 way interactions involving ov, wealth and edu. You almost always want to include the lower level terms when running a model with interactions. So, if you thought those 2 models, as written, were equivalent, they are not.

Incidentally, if you have Stata 11, you may wish to use factor variables instead of computing your own interaction terms. This is especially true if you want to run the margins post-estimation command.


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Richard Williams, Notre Dame Dept of Sociology
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