As has been amply demonstrated in many research areas, individual differences account for the bulk of the variance of phenomena studied. Depending on the researcher's theoretical orientation and training, variance due to individual differences is either treated as error or serves as the focus of study. Probably no other author has discussed this issue with greater insight and clarity than Cronbach (1957) in his classic paper on the two disciplines of scientific psychology. Because Cronbach addressed broad differences between two approaches to research, we believe you will benefit from reading his statement even if your field of study is not psychology. Briefly, Cronbach distinguished between experimentalists and correlationists. 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."