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Re: st: problem with factor variables in RE probit


From   Richard Williams <richardwilliams.ndu@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: problem with factor variables in RE probit
Date   Tue, 27 Sep 2011 10:29:32 -0400

At 07:41 AM 9/27/2011, Liberini, Federica wrote:
Dear listers,

I am having problems using the factor variables in a RE probit model.

My "group" variable is a categorical that can take up to 3 different
values. My "X" variable is a continuous.
What I did was to compare two regressions:

tab group, gen(group)
gen gr1_x=group1*x
gen gr2_x=group2*x

xtprobit y x group1 group2 gr1_x gr2_x xbar zbar, re

compared to

xtprobit y b3.group##c.x xbar zbar, re

(the xbar and zbar variables are for the RE specification)

Now, before running the two regressions I made sure that the virtual
variables generated by b3.group##c.x were identical to the varlist x
group1 group2 gr1_x gr2_x . I did this by typing

list x group1 group2 gr1_x gr2_x  b3.group##c.x in 1/100

and they were!

So why is that the two regression command return different outcomes?

You don't say how the outcomes are different. Are the fits identical but the parameterizations are different? Personally, I tend to avoid the ## notation and just try to write things the same way I would if I had separate variables. That way I feel more sure of what I am asking for. So, I would rewrite your first command as

xtprobit y x b3.group b3.group#c.x xbar zbar, re

If that doesn't work then provide more detail as to how the results are different.


-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
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EMAIL:  Richard.A.Williams.5@ND.Edu
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