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Re: st: Testing for interaction the right way


From   Maarten Buis <maartenlbuis@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Testing for interaction the right way
Date   Mon, 30 Jul 2012 16:18:40 +0200

My bad, I did not see that bwtgestage_sd was continuous, use
-i.motherland2##c.bwtgestage_sd-. For more see: -help fvvarlist-.

-- Maarten

On Mon, Jul 30, 2012 at 4:05 PM, Amal Khanolkar <Amal.Khanolkar@ki.se> wrote:
> Hi Maarten,
>
> Thanks for the suggestions.
>
> When I tried to use the '##' to indicate an interaction I got the following error message:
>
> . regress syst i.motherland2##bwtgestage_sd age byear i.conscript_office bmi height_cons i.education if multibirth==1
> bwtgestage_sd:  factor variables may not contain noninteger values
> r(452);
>
> end of do-file
>
> Have I left out something, or should it be done differently? (bwtgestage_sd is a continuous variable).
>
> Thanks,
>
> /Amal.
>
> ________________________________________
> From: owner-statalist@hsphsun2.harvard.edu [owner-statalist@hsphsun2.harvard.edu] on behalf of Maarten Buis [maartenlbuis@gmail.com]
> Sent: 30 July 2012 15:53
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: Testing for interaction the right way
>
> On Mon, Jul 30, 2012 at 2:52 PM, Amal Khanolkar  wrote:
>>  eststo: xi: regress syst i.motherland2*bwtgestage_sd age byear i.conscript_office bmi height_cons i.education if multibirth==1
>> 1. I would like to know if I have included the interaction term (i.motherland2*bwtgestage_sd) in the right way?
>
> You are much better off using Stata's factor variable notation:
> i.e.leave the -xi- prefix away and use -i.motherland2##bwtgestage-
>
>> 2. I would have expected more of the interaction tests to be significant (given the results by ethnic group in the first regression model above).
>
> No, a model without interactions cannot tell you much about the
> presence or absence of interaction effects.
>
>> 3. I also ran the bwt-syst. BP regression 'stratified' by ethnic group. But then I assume it's not sensible to text for interaction after each stratified analysis separately as it does not make sense, since the bwt-syst. BP association is now a comparison only between one ethnic gropu and the reference group?
>
> It is not a matter of not being sensible, it is a matter of being
> impossible: An interaction effect means that you think that effects
> are different between ethnic groups, i.e. it is a comparison. You
> cannot compare if you only have one group, which is what you get when
> you estimate separate models.
>
> Hope this helps,
> Maarten
>
> --------------------------
> Maarten L. Buis
> Institut fuer Soziologie
> Universitaet Tuebingen
> Wilhelmstrasse 36
> 72074 Tuebingen
> Germany
>
>
> http://www.maartenbuis.nl
> --------------------------
>
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-- 
--------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany


http://www.maartenbuis.nl
--------------------------

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