gt.bpm can be used to test the hypothesis of absence of endogeneity, correlated model equations/errors or non-random sample selection
in binary bivariate probit models.
The gradient test was first proposed by Terrell (2002) and it is based on classic likelihood theory. See Marra et al. (in press) for full details.
It returns a numeric p-value corresponding to the null hypothesis that the correlation, θ, is equal to 0.
This test's implementation is only valid for bivariate binary probit models with normal errors.
Maintainer: Giampiero Marra firstname.lastname@example.org
Marra G., Radice R. and Filippou P. (in press), Regression Spline Bivariate Probit Models: A Practical Approach to Testing for Exogeneity. Communications in Statistics - Simulation and Computation.
Terrell G. (2002), The Gradient Statistic. Computing Science and Statistics, 34, 206-215.
## see examples for SemiParBIVProbit