Re: st: Marginal Effects after ivtobit

["Brian P. Poi" <bpoi@stata.com>]

On Sat, 17 Dec 2005, Alejandro Lopez-Feldman wrote:

> I want to calculate marginal effects afterivtobit using mfx but I have a
> question about the standard errors.
>
> The Stata manual ([R] page 552) states that the option "force" has to be
> used otherwise mfx refuses to calculate standard errors. I have done this
> and obtained standard errors.  My question is are those standard errors
> correct or should I worry that they don't account for the fact that tobit
> is using a predicted value?
>
> Thanks,
> Alejandro
Alejandro,

Using the "force" option with -mfx- after -ivprobit- or -ivtobit- is fine.

The warning message you get when you do not use that option arises from 
the fact that the endogenous variable appears on the righthand side of the 
structural equation and on the lefthand side of the equation that relates 
it to the instruments.  In this case, once we've consistently estimated 
the parameters of the structural equation and their covariance matrix, we 
can go ahead and get marginal effects and their standard errors in the 
usual way.

   -- Brian
   -- bpoi@stata.com
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