ABSTRACT
Individual differences have been an annoyance rather than a challenge to the experimenter. His goal is to control behavior, and variation within treatments is proof that he has not succeeded. Individual variation is cast into that outer darkness known as “error variance.” For reasons both statistical and philosophical, error variance is to be reduced by any possible device. You turn to animals of a cheap and short-lived species, so that you can use subjects with controlled heredity and controlled experience. You select human subjects from a narrow subculture. You decorticate your subject by cutting neurons or by giving him an environment so meaningless that his unique responses disappear. You increase the number of cases to obtain stable averages, or you reduce N to I as Skinner does. But whatever your device, your goal in the experimental tradition is to get those embarrassing differential variables out of sight. (Cronbach, 1957, p. 674.)
