The Effect of Schooling and Ability onAchievement Test Scores by Hansen, Heckman.

Our model of test scores is based on an extension of the factor analysis model used in psychometrics(see e.g., Lord and Novick, 1968). Test score T (s) is a manifestation of latent ability f mediated byschooling:
T (s) = μ(s) + λ(s)f + ε(s)