Dear stata-list users,
I'd appreciate a piece of advice. I've got a two-level hierarchical
dataset: patients at the lower level and GPs at the higher level. Say I
want to assess the relationship between medical expenditures and patient age
controlling for GP years of training as follows:
Expenditures = f(PatientAge, GPtraining)
I can use cluster robust standard errors, to relax the assumption of
independence of observations on a GP patient list:
reg Expenditures PatientAge GPtraining, cluster(gpid)
If I do so, is it valid to comment on the coefficient on the GPtraining
variable in the normal way?
If not, I could use fixed effects specification:
xtreg Expenditures PatientAge GPtraining, fe i(gpid)
But when I do this, the GPtraining variable gets dropped, so instead I've
done this:
xi i.gpid
reg Expenditures PatientAge GPtraining I_gpid_2 - Igpid_455
This seems to work. However I can't use xthausman to compare a random
effects and a fixed effects specification. Can I modify xtreg, fe in some
way so that it drops one GP dummy rather than all the GP-level variables, so
that I can apply xthausman?
Brendan McElroy
University College Cork
Western Road
Cork
Ireland
Tel: +353 21 490 3522
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