“…+ vft where time subscripts as well as unobserved individual-specific effects, a,, are introduced, which may be correlated with Dit, and where V-, are random error terms that are 5 This error measurement or misclassification problem is also raised in the economics of education literature. See, for example, Ashenfelter and Krueger (1994); Black, Berger, and Scott (2000); and Miller, Mulvey, and Martin (2006 (3) D^D.+^Hi where T'it represents a time-independent unsystematic error and ^ represents a timepersistent error. Substitution of equation (3) in equation (2) leads to equation (4) and clarifies the possible correlations between the error term and the language variable: i™, = **ß + (4 -^ -S,-)y + ai + vü = xü$ + Düy + (ai-yEsi) + (vü-yr'ü)…”