Notice: On April 23, 2014, Statalist moved from an email list to a forum, based at statalist.org.

Re: st: Followup to: simple slopes between continuous variable and dichotomous moderator

 From David de Jong <[email protected]> To [email protected] Subject Re: st: Followup to: simple slopes between continuous variable and dichotomous moderator Date Thu, 20 Jun 2013 12:57:27 -0400

```Hi Maarten and Richard,

main effect of foreign solves the problem. And Maarten, thank you for
the suggestions for additional strategies and explanations, that helps
a lot.

I appreciate your help very much!

Best,

David de Jong

On Thu, Jun 20, 2013 at 4:11 AM, Maarten Buis <[email protected]> wrote:
> That example should have included the main effect of foreign. There
> are various ways in which you can obtain the simple slopes after such
> an interaction effect:
>
> *------------ begin example --------------
> sysuse auto, clear
> reg price c.mpg##i.foreign
> margins foreign, dydx(mpg)
>
> // or
>
> reg price c.mpg#i.foreign i.foreign
> // margins is now superfluous as you can directly
> // read these numbers from the output of -regress-
> margins foreign, dydx(mpg)
>
> // or
>
> reg price c.mpg#ibn.foreign i.foreign, hascons
> // margins is now superfluous as you can directly
> // read these numbers from the output of -regress-
> margins foreign, dydx(mpg)
>
> // or
>
> bys foreign: reg price mpg
> *------------- end example ---------------
>
> I minor detail is that in the last model the standard errors are
> different from the standard errors in the previous models. This
> difference comes from the fact that in the first two models the error
> variance is assumed to be the same for foreign and domestic cars,
> while in the last model separate error variances are estimated for
> both types of cars. If this worries you, just specify the
> -vce(robust)- option for the model with interaction terms.
>
> Hope this helps,
> Maarten
>
>
> On Thu, Jun 20, 2013 at 4:53 AM, David de Jong <[email protected]> wrote:
>> Dear STATA folks,
>>
>> I’m trying to figure out how to plot simple slopes to unpack an
>> interaction between a continuous and categorical variable in a
>> regression analysis. Let’s say I want to look at the simple slopes for
>> price on mpg, for both foreign and domestic cars.
>>
>> In my searches, I came across this exchange from a few years ago:
>>
>> --- On Tue, 1/2/11, Rodrigo Isidor wrote:
>>
>>> Has anyone written a program to calculate simple slopes for
>>> interactions between a continuous variable and dichotomous
>>> moderator in linear regression analysis? To my knowledge,
>>> sslope from Jeffrey Simons is just for continuous
>>> variables.
>>
>> There is no need to write a program for that. You can use Stata's
>> factor variable notation for it. In the example  below the first slope
>> for mpg is for domestic cars and the second is for foreign cars.
>>
>> *------------ begin example --------------
>> sysuse auto, clear
>> reg price c.mpg#i.foreign
>> *------------- end example ---------------
>>
>> Hope this helps, Maarten
>>
>> [end previous exchange]
>>
>> I tried that approach, as well as using -margins-, with the following:
>>
>> margins i.foreign, dydx(mpg)
>>
>> These two approaches produced very different results. The –margins-
>> approach gives me results more consistent with results from simply
>> splitting the file by foreign and looking at the correlation between
>> price and mpg. I’m sure that I’m missing something. Can anyone advise
>> as to why these procedures are giving me such different results, and
>> if I’m doing either incorrectly?
>>
>> Thank you,
>>
>> David de Jong
>>
>> ~
>> Department of Clinical and Social Sciences in Psychology
>> University of Rochester
>> 498c Meliora Hall
>> Rochester, NY 14627
>>
>> *
>> *   For searches and help try:
>> *   http://www.stata.com/help.cgi?search
>> *   http://www.stata.com/support/faqs/resources/statalist-faq/
>> *   http://www.ats.ucla.edu/stat/stata/
>
>
>
> --
> ---------------------------------
> Maarten L. Buis
> WZB
> Reichpietschufer 50
> 10785 Berlin
> Germany
>
> http://www.maartenbuis.nl
> ---------------------------------
>
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/faqs/resources/statalist-faq/
> *   http://www.ats.ucla.edu/stat/stata/

--
~
Department of Clinical and Social Sciences in Psychology
University of Rochester
498c Meliora Hall
Rochester, NY 14627

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

• References: