st: Calculating margins after biprobit

["Wakeman, Simon" <Simon.Wakeman@esmt.org>]

I am investigating the effect of a binary variable X on a binary variable Y, where X is potentially endogenous. I estimated a biprobit:
 biprobit (Y = X ) (X = Z). 

However, when I use the following margins command
 margins, dydx(X) over(X) 
I get the error message:
"default prediction is a function of possibly stochastic quantities other than e(b)"

I also tried mfx but it does not seem to produce a marginal effect for X.

I would much appreciate if anyone can tell me how to obtain the marginal effect of X on Y.

Thanks for your help.

Simon


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