“…, θ J ,∆ n could be the first order conditions for a semiparametric weighted least squares estimator of index parameters as in Ichimura and Lee (1991) or when J = 1,∆ n could be the first order conditions for semiparametric weighted least squares or maximum likelihood estimators as those in Ichimura (1993) and Klein and Spady (1993), respectively. Similarly, if X := (X ⊤ 1 , X ⊤ 2 , Z ⊤ 1 , Z ⊤ 2 ) ⊤ and W (X) = (Z ⊤ 1 θ 1 + X ⊤ 2 θ 2 , X 2 − g(Z 1 , Z 2 )), then∆ n could be the first order conditions for semiparametric weighted least squares or maximum likelihood estimators that uses 'control function' approaches as in Escanciano, Jacho-Chávez and Lewbel (2011) and Rothe (2009) respectively. Alternatively, if W (X) = X 1 ⊂ X,∆ n also has the form of test statistics designed to test nonparametrically the significance of a subset of covariates as in Delgado and González Manteiga (2001).…”