st: Dtobit mfx and Mcdonal and Moffit decomposition - tobit

["olivier lohest" <olohest@hotmail.com>]

Hi

I am estimating a tobit model for migration (cross section)

I am using the dtobit "procedure"* to compute the unconditionnal expected 
value of the marginal effect but also to decompose this effect into two 
components : the marginal effect of the conditional to being unconsered and 
the marginal effect of the probability of being unconsored....The dtobit 
command is useful to compute quieckly this result.


My question is :

when I compare the marginal effect of the unconditionnal expected value and 
the sum of the marginal effect of the conditional to being unconred and of 
the probability of being unconsored, I don't have the same result.... the 
sum of these two components is not equal of the marginal effect of the 
unconditionnal expected value. I am using stata 8....

When I am looking to the empirical literature, I found also the same 
difference if the people have used stata

Do you any explications for these differences

Thank your for your help


best

Olivier

* I have also trying to compute these marginal effects via the command mfx 
(I obtain the same result)

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