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RE: st: question about the interaction term


From   ZHVictor <victerzj2@hotmail.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   RE: st: question about the interaction term
Date   Thu, 25 Apr 2013 04:12:58 +0000

Dear David,
Thank you for your response. I understand what you are saying. 
In B=0 case, I have a significant slope for A
In B=1 case, the slope of A becomes coefficient of A+coefficient of A*B, however it becomes insignificant.
A*B is an interaction term. I have an insignificant coefficient of A*B. That means the coefficient of A*B is like 0
Thus, in B=1 case, the slope of A is like coefficient of A+0. Therefore, I am expecting in B=1 case, the slope of A should be also significant as in B=0 case. 
That is where I am confused.
Thank you!



Vic
----------------------------------------
> Date: Wed, 24 Apr 2013 23:52:05 -0400
> Subject: Re: st: question about the interaction term
> From: dchoaglin@gmail.com
> To: statalist@hsphsun2.harvard.edu
>
> For the moment, let's set aside the question of whether the
> interaction term is significant.
>
> We can write out the model in the two cases (B=0 and B=1).
> If B=0, you have y = constant + A
> If B=1, you have y = constant + A + B + A*B
> That is, when B=0, the model provides an intercept and a slope against
> A; and when B=1, it provides a separate intercept (constant +
> coefficient of B) and slope against A (coefficient of A + coefficient
> of A*B). The test that you quoted says that this slope does not
> differ significantly from 0. That result is not inconsistent with
> having a negative coefficient for A that differs significantly from 0.
>
> David Hoaglin
>
> On Wed, Apr 24, 2013 at 10:46 PM, ZHVictor <victerzj2@hotmail.com> wrote:
> > I am running probit model with an interaction term. The regression is like this
> > y=constant+A+B+A*B
> > A is a continuous variable and B is a dummy variable with either zero or one.
> > A has a significant and negative coefficient but A*B has an insignificant and positive coefficient.
> > However, when I write "test A+A*B=0" on stata, stata reports "Prob > chi2 = 0.1718". That means when B=1, the coefficient of A becomes insignificant. However, When B=0, the coefficient of A is significant.
> > However, I think if interaction term is insignificant, A should have the same negative and significant coefficient no matter whether B is zero or one.
> > I feel the question is a little bit stupid but I just can not figure out why.
> > Any comment is welcome.
> > Thank you in advance!
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