Probabilistic inferences,discrimination, and stimulus interference in comparative judgement

Comparative judgements of successive stimuli involve constant errors. Here, two theories of these errors are considered. For the wave theory of discrimination, constant errors directly originate from discrimination operations. For the inferential theory, the same errors are due to probabilistic inferences drawn after discrimination. Previous studies suggest that the second of two successively compared stimuli has more influence on the comparative response. The results of the present experiment, in which successive lines as comparison stimuli were used, confirm this suggestion. The wave theory does not explain these results, while the inferential theory explains them in terms of probabilistic inferences. Analysis of response frequencies and response times indicates that there was a response bias in favour of the longer response. Interfering lines appearing before the first comparison line significantly affected response frequency, response time, and discriminability, whereas interpolated lines significantly affected only response frequency and response time. The inferential theory does not explain these effects on response time, while the wave theory explains them as due to modifications of response bias. These results suggest the possibility that the two theories describe independent sources of constant errors.