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From |
Amal Khanolkar <Amal.Khanolkar@ki.se> |

To |
"statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu> |

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 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 -------------------------- * * 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/ * * 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/

**Follow-Ups**:**Re: st: Testing for interaction the right way***From:*Maarten Buis <maartenlbuis@gmail.com>

**References**:**st: Testing for interaction the right way***From:*Amal Khanolkar <Amal.Khanolkar@ki.se>

**Re: st: Testing for interaction the right way***From:*Maarten Buis <maartenlbuis@gmail.com>

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