st: Test the equality of regression coefficients

[Simon Moore <simoncmoore@gmail.com>]

Dear Statalist,

I'm trying to test the test the equality of regression coefficients for 
two models.  Looking at:

http://www.stata.com/support/faqs/stat/testing.html

I can see that the solution should be simple, but I'm stuck.

I'm modeling binary choice data across a series of trials per person to 
see how features of the choice environment effects choice for three 
groups.  I started with the simplest model:-

tsset id trial
xi: xtlogit choice i.group*var1 var2, i(id) nolog re

Each group completed the task under two conditions and I want to compare
coefficients on the i.group*var1 interaction.  So according to the above 
stata.com page I should append the data and then run something like

tsset id trial
xi3: xtlogit choice i.condition*i.group*var1 var2, i(id) nolog re

and then something like

test _IgroupXcondition_2 - _IgroupXcondition_3 = 0
local sign_ag = sign(_b[_IgroupXcondition_2] - _b[_IgroupXcondition_3])
display "H_0: group1 coef >= group2 coef. p-value = " 
normal(`sign_ag'*sqrt(r(chi2)))

But of course, I get

tsset id trial
repeated time values within panel

It seems like there should be a very simple way around this - but I'm 
stuck.  So any hints would be greatly appreciated.

Regards
Simon


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