Comparison is not just subtraction:: Effects of time- and space-order on subjective stimulus difference

In five experiments, participants made comparative judgments of paired successive or simultaneous stimuli. Time- or space-order errors were obtained, which varied with the interstimulus interval (ISI) or stimulus duration, as well as with the stimulus level. The results, in terms of scaled subjective differences, are well described by Hellstrom's (1979) sensation-weighting model. With successive presentation, in comparisons of line length and tone loudness, the first stimulus had the greater weight in determining the subjective difference for short ISIs, the second for longer ISIs. In comparisons of duration (auditory and visual), the second stimulus had the greater weight. For simultaneously presented line lengths, the left stimulus had the greater weight.