Range Effects in Experiments on People

Abstract

Range effects occur when each person receives more than one experimental condition. Sometimes the range effects are studied deliberately; sometimes they are considered traps to be avoided; often they go unrecognized. Examples from studies in psychophysics, perception, learning and memory, reaction times, and motor skills demonstrate that range effects are critical to the measurement of almost any kind of human behavior. The results of a within-subjects design should always be regarded with suspicion. No psychologist should use a withinsubjects design, except for a special purpose, without combining it with a separate-groups design. In a within-subjects experimental design, each person performs more than one condition. The latin square of Figure 1 is an example. A withinsubjects design is said to take care of the effects of practice, but it does not do so. The difference between the two conditions A and B depends partly on the two conditions compared, and partly on the other experimental conditions, C and D, in the design. No clear interpretation of the difference between A and B is possible. It is not possible to tell how much is main effect, and how much is range effect due to including C and D in the experimental design.