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help for complogit                                              (Glenn Hoetker)
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Compare coefficients across logit equations

        complogit varlist , group(varname) [ numerator(varname)
               denominator(varname) specific(varname) ]


Description


complogit allows for the accurate comparison of coefficients across
groups in a logit equation by automating the calculations discussed in Hoetker 
> (2004; 2007).  
This help file will make little sense without reference to those articles.


In its basic application, it performs the requested logit equation on
all observations, on cases where the  group variable=0 and on cases 
where it equals 1.  It then performs a logit for all observations, allowing
residual variance to differ across groups. It then tests whether residual 
variation differs across groups using a likelihood ratio and Wald chi-square 
test. Lastly, it uses a likelihood ratio test to test the null hypothesis 
that the true coefficients are the same across groups versus the alternative 
that at least one coefficient differs.


Optionally, the user may request a test of test of the null hypothesis that the
>  
true coefficients are the same across groups versus the alternative that 
a specific coefficient differs. To do so, specify the desired coefficient
in the specific option.  Note that this test assumes that all other
coefficients are equal across groups.


Optionally, the user may request a test that the ratio of two coeffiients
is the same across groups.  To do so, specify the desired variables in the
numerator and denominator options.


Options

See above


Remarks


Version 8 of Stata is required.  There is minimal error checking. 
complogitml.ado contains the maximum likelihood routine and is of little direct
use.  It is based on code contained in Allison (1999). Users may also wish to s
> ee the  oglm command
of Richard Williams and his associated papers.

References

Allison, P.D. 1999. Comparing logit and probit coefficients across groups. 
 SMR/Sociological Methods & Research. 28(2): 186-208.

Hoetker, G. 2004. Confounded coefficients: Accurately comparing logit 
and probit coefficients across groups.  Working paper. 
http://www.business.uiuc.edu/ghoetker/documents/Hoetker_comp_logit.pdf

Hoetker, G. 2007. The use of logit and probit models in strategic management re
> search: Critical issues.
 Strategic Management Journal. 28(4): 331-343. 


Examples

                . complogit y x1 x2, group(type)

                . complogit y x1 x2, group(type) num(x1) den(x2)

                . complogit y x1 x2, group(type) sp(x1)


Author

Glenn Hoetker, College of Business, University of Illinois at Urbana-Champaign
Email: ghoetker@uiuc.edu