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st: sandwich estimator for two-stage models

From   "Meike Wollni" <>
Subject   st: sandwich estimator for two-stage models
Date   Sat, 5 Nov 2005 17:55:19 -0500 (EST)

Dear stata-list users:
I am estimating a two-stage model: Probit in the first stage, gls in the

I derive the sandwich estimate of variance to obtain corrected standard
errors for the second stage following the description in:

Hardin, J.W.: The robust variance estimator for two-stage models. The
Stata Journal (2002) 2, No. 3, pp. 253-266

My problem is that I want to include two generated regressors in the
second stage.
P = bX + cS_hat + dC_hat + e

where P-hat and C_hat are generated from two seperate probit models in the
first stage. (X is a matrix of exogenous vars, and b, c, d are unknown
vectors of parameters, and e is an error term)

Can anybody help me on how to adjust Hardin's equation for the sandwich
estimate of variance to account for a second generated regressor in the
second stage? Any suggestions will be very much appreciated.

Thank you

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