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st: dummy interaction


From   "Mol, M.C." <m.c.mol@vu.nl>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: dummy interaction
Date   Thu, 2 May 2013 13:37:43 +0000

Dear all,
I seem to be missing something interpreting the interaction between to dummy variables. I always understood that it should not really make any difference whether or not you include a dummy variable as being continuous or factor.  This seems confirmed by running reg Y i.dummy1##i.dummy2 which gives me the same results as reg Y c.dummy1##c.dummy2, which again gives me the same results as running reg Y i.dummy1 i.dummy2 i.dummy1#i.dummy2.

However, when I only include the main effect of 1 of the dummies and run reg Y i.dummy1 i.dummy1#i.dummy2 the results differ from reg Y c.dummy1 c.dummy1#c.dummy2

Would anyone care to explain to me what I am missing
Sincerely
Mischa

PS: The reason for running without the main effect of the second dummy is t=
hat I have multiple measurements per respondent and the second dummy only v=
aries between respondents and not in time (and therefore gets omitted).

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