A memory model of sequential effects in scaling tasks.

Subjects judge successive stimuli to be overly similar in psychophysical scaling tasks. This is called assimilation. They also tend to judge each stimulus as overly different from more previous events. This is called contrast. To examine a two-stage linear model of these sequence effects, we asked subjects to judge the relative intensity of successive tones. In support of the model, responses again depended lawfully on prior events. These memory effects occur in a variety of scaling tasks and are consistent with two assumptions: (a)Successive events assimilate in memory, and (b) subjects compare each stimulus to a collection of memories of prior events to generate a response. The trial-by-trial analysis used to test the model also showed that even in magnitude-estimation studies, equal stimulus ratios do not result in equal response ratios, except on average. This article suggests that examinations of trial-by-trial performance might be useful in studying memory and judgment processes.