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st: conditional logit problem

Subject   st: conditional logit problem
Date   Fri, 27 Mar 2009 09:42:14 +0100 (CET)

Dear statalisters,

I have  a dataset with firms, to which is attributed a number called
"siren", and characteristics of thses firms (exportations, size, value
added...). I have too the country in which they decide to invest, and
characteristics of this country (GDP, population, tax rate, labor

So my database looks like this (it is an example with 3 firms which choose
one unique country):
siren	   va	   country     tax	    GDP          choice
17251067	   94997	   Spain	      .35	    7.77e+11       1
17251067	   94997	   Brazil	      .34	    4.86e+11       0
17251067	   94997	   Italy        .38    1.33e+12      0

3.025e+08   28142	   Brazil	      .34	    4.86e+11       1
3.025e+08   28142    Spain	      .35	    7.77e+11       0
3.025e+08   28142    Italy        .38    1.33e+12       0

3.026e+08	   75758    Italy        .38    1.33e+12       1
3.026e+08	   75758    Spain	       .35    7.77e+11       0
3.026e+08	   75758 	   Brazil	       .34    4.86e+11       0

When I run a conditional logit with only countries'characteristics as
regressors (***clogit choice country lntax lnGDP, group(siren)**, where
lntax and lnGDP are the log of tax and GDP), it is OK.
But what I don't understand is this point:

-When I add value added as a control variable (***clogit choice country
lntax lnGDP va, group(siren)***), it works, even if the coefficient for va
is strange.
-When I add the log of value added as a control variable ((***clogit
choice country lntax lnGDP lnva, group(siren)***), STATA refuses to
provide results and write:"outcome does not varies in any group".

So I wonder why it is ok in a case and not in the other else which is very
similar, and I don't know what kind of logit to run which allows me to mix
the two types of variables.

Thank you very much for help,


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