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st: limited dependent variable & cross equation restrictions


From   owner-statalist@hsphsun2.harvard.edu
To   statalist@hsphsun2.harvard.edu
Subject   st: limited dependent variable & cross equation restrictions
Date   Tue, 20 Aug 2002 11:08:40 +1000

Hi list,
I'd like to understand how to deal with the 2 issues below and would 
welcome any references/feedback you may be able to suggest.

I run the regression (coefficients omitted):

Fijt = constant + Nijt + Djt + Djt*Nijt + control variables + error

where Fijt = share of foreigners of skill group i in region j at time t
Nijt = corresponding share amongst natives
Djt = dummy indicating whether region j is abundant in skill i relative to 
all regions at time t. Djt = 1 is Nijt > average share of skill i amongst 
natives across all regions at time t. Dji = 0 otherwise.

I estimate that regression using fixed-effects.

ISSUES:

1. Fijt is a limited dependent variable whose value ranges between 0 
and 1. Do I really need to transform my variable so that its value 
becomes unbounded?

2. I have 3 skill groups and hence have an add-up constraint (i.e. the 
sum of Fijt across the i is 1). Is it legitimate to estimate separately the 
above regression for the 3 skill groups on the understanding that the 
non-linearity given by the dummy and the interaction term makes it 
virtually impossible to impose an add-up constraint across the Djt and 
Dij*Nijt ? In other words, can i simply estimate the regression above for 
the 3 groups on the assumption that any add-up restriction is picked up 
by the error term (hence I should only worry about heteroskedasticity)?

Many thanks for your help,
Max

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