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# Re: st: Testing the difference between coefficients with interaction variables

 From Tirthankar Chakravarty <[email protected]> To [email protected] Subject Re: st: Testing the difference between coefficients with interaction variables Date Fri, 26 Aug 2011 23:28:01 -0700

```Here is an example:

/****************************/
sysuse auto, clear
regress price c.mpg##i.rep78
testparm c.mpg#i.rep78
/****************************/

This tests whether or not the coefficient on "mpg" is significantly
different across categories of "rep78". In this case, you are unable
to reject the hypothesis that they are indistinguishable from the
coefficient of "mpg" for the base group.

T

On Fri, Aug 26, 2011 at 11:12 PM, Michael Crain <[email protected]> wrote:
> I know testing the difference between coefficients of variables is
> straightforward:
>
> reg y x1 x2 x3
> test x1 = x2 = x3
>
> Is the test as straightforward for examining coefficients in models
> with interactions variables?
>
> reg y x1 x2 x3 x1*x2 x1*x3
>
>   where:
>
>     x2 and x3 are dummy variables that identify groups
>
>     the base group is when x2=0 and x3=0, group 2
>     is when x2=1 and x3=0, group 3 is when x2=0 and x3=1
>
> I am interpreting the effect of the base group as x1's coefficient. The
> effect of group 2 is the sum of coefficients of the x1 and x1*x2
> variables. Lastly, the effect of group 3 is the sum of coefficients of
> the x1 and x1*x3 variables.
>
> Now, I want to examine whether the effects of the three groups are
> different.
>
> Is the test:
>
> test x1 = x1*x2 = x1*x3
>
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>

--
Tirthankar Chakravarty
[email protected]
[email protected]

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/
```

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