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# Re: st: Test for significance in Interaction model

 From Lukas Borkowski To statalist@hsphsun2.harvard.edu Subject Re: st: Test for significance in Interaction model Date Mon, 4 Jun 2012 21:33:45 +0200

```Dear David,

thanks for your help. I just ran a F-test for joint significance of b2 and b3. Thats what I meant by the combined effect of x2 and x1*x2. So you perfectly answered by questiont! Thanks!

#
Lukas Borkowski

Am 03.06.2012 um 19:48 schrieb David Hoaglin:

> Dear Lukas,
>
> It would help if you told us more about what you mean by "the combined
> effect of x2 and x1*x2 for the case of x1=1."
>
>
> b0 is the mean of y for x1 = 0, adjusting for the contribution of x2.
> In this model, when x1 = 0, you have the line y = b0 + b2x2, so b0 is
> its intercept.
>
> b1 is the difference between the mean of y for x1 = 1 and the mean of
> y for x1 = 0, adjusting for the contribution of x2.
>
> b2 is the slope of y against x2 when x1 = 0
>
> b2 + b3 is the slope of y against x2 when x1 = 1.
>
>
> It is often of interest to test whether b3 = 0 (i.e., whether the
> slope of y against x2 is the same for x1 = 1 and x1 = 0) and, if so,
> to test whether b2 = 0 and b3 = 0 (i.e., no slope against x2).
>
> David Hoaglin
>
> On Sun, Jun 3, 2012 at 7:04 AM, Lukas Borkowski <lukasbork@hotmail.com> wrote:
>> Dear all,
>>
>> I am a quite new stata user and I am trying to find a way to check whether the combined effect of my interaction effect is significant.
>> My model can be simplified as y = b0 + b1x1 + b2x2 + b3x*1x2.  x1 is a dummy variable and x2 a  variable. I would now like to check whether the combined effect of x2 and x1*x2 for the case of x1=1 is significant. But I am not sure how. I used -test x2 x1*x2- and -testnl _b[x2]*_b[x1*x2] = 0- but I have absolutely no idea if that is correct or not. Could you please help me? Thank you!
>
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