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Re: st: question about the interaction term


From   Maarten Buis <maartenlbuis@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: question about the interaction term
Date   Thu, 25 Apr 2013 10:12:08 +0200

On Thu, Apr 25, 2013 at 9:32 AM, ZHVictor wrote:
> So that means whenever I have the similar regression, I should use "test A+A*B=0" to double check, rather than only look at the interaction term.

No, that is exactly opposite of what the main point of that article is.

> Thus, for B=0 case, I should only look at the p-value of the coefficient of A to see whether the coefficient of A is significant.
> However, for B=1 case, I should actually test whether A+A*B is significant (use test A+A*B=0). If A+A*B is insignificant different from zero, I should say A has on effect on Y when B=1, even if the interaction term is insignificant.
> Is what my understanding correct?

No, the trick is to work out exactly what the null hypothesis is that
you want to test and create a test that tests exactly that null
hypothesis.  An interaction term measures exactly what an interaction
term measures and if that is what you want to know then that is
enough.

> One more question is if the coefficient of A is -0.4 and the coefficient for the interaction is 0.2, so the coefficient of A in B=1 case should be -0.4+0.2=-0.2 but not -0.4+0=-0.4. Is that correct?

I don't understand that question, where did the 0 come from?

-- Maarten

---------------------------------
Maarten L. Buis
WZB
Reichpietschufer 50
10785 Berlin
Germany

http://www.maartenbuis.nl
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