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st: Setting the base group in dummy variable interaction terms


From   "Tim Burnett (ECO)" <Tim.Burnett@uea.ac.uk>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Setting the base group in dummy variable interaction terms
Date   Sun, 30 Sep 2012 11:04:35 +0000

Dear all,

I have a question which may b trivially easy to answer but unfortunately I can't find it.

I am using a random effects probit model as follows:

xtprobit  switched ib4.service ib1.supplier ib2.duration  bundled numberservs i.supplier#i.service  i.bundled#ib1.supplier i.bundled#ib4.service  ib1.income ib1.employment i.children i.gender ib1.education ib2.age if INCLUDEVAR==1, re

Which obviously includes many dummy variables.  I am using the prefix ib#. to set the base category for the dummy variables for the standalone dummies. Unfortunately this isn't working for the dummy interaction terms which I would like to be based upon the excluded categories of the standalone variables (4 for service, 1 for supplier) since in these cases the excluded category has specific relevance in terms of understanding the results.. 

Placing, respectively, ib1 and ib4 in the command above doesn't seem to do any harm, but Stata just ignores it as follows:

note: 1.bundled#6.supplier omitted because of collinearity
note: 1.bundled#4.service omitted because of collinearity

It ignores my base category request.

Any suggestions?

Many thanks,

Tim

--

Tim Burnett
PhD Researcher
Department of Economics & ESRC Centre for Competition Policy
University of East Anglia

Email: tim.burnett@uea.ac.uk
Phone: +44 (0) 7793 116 522
Skype: timb318

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