“…In other words, in terms of the augmented model described above, we get a better estimator of θ 1 when we use the estimated θ 2 in the second step than if we used the true θ 2 . This phenomenon has been discussed by Wooldridge (1999Wooldridge ( , 2001Wooldridge ( , 2002bWooldridge ( , 2007, and it has also been noted in a number of previous works, including Pierce (1982), Rosenbaum (1987), Imbens (1992), Robins et al (1992), Robins and Rotnitzky (1995), Hirano et al (2003), Henmi and Eguchi (2004) and Hitomi et al (2008). This is puzzling because knowledge of θ 2 , if properly exploited, cannot be harmful.…”