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RE: st: Testing for interaction the right way

From   Amal Khanolkar <>
To   "" <>
Subject   RE: st: Testing for interaction the right way
Date   Mon, 30 Jul 2012 14:05:10 +0000

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 if multibirth==1
bwtgestage_sd:  factor variables may not contain noninteger values

end of do-file

Have I left out something, or should it be done differently? (bwtgestage_sd is a continuous variable).



From: [] on behalf of Maarten Buis []
Sent: 30 July 2012 15:53
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 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 L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen

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