A distinction is discussed between manifest and latent concepts on the one hand and observable and unobservable psychological processes (of which learning is one) on the other. Previous attempts at measurement by Hull and Bjorkman are briefly reviewed. An analysis is presented of improved performance in terms of the latent variables of the Law of Comparative Judgment. Finally, the author's method for successive intervals scaling is applied to some paired associate learning data. (RB is a reprint from the journal.)
