[Primacy effects in psychophysical judgements]

In the first of a series of three experiments, two groups of 10 subjects judged either the bottom half or the top half of a series of 16 squares according to their size. After two presentations with the pretraining series, 10 trials with the total series followed. The initial judgments of the pretraining stimuli showed a marked tendency to persist all through the experiment, thus demonstrating a primacy effect. The effect was not completely removed by instructions to modify the judgment scale (Experiment 2). In the third experiment, the number of categories was varied. With a larger number of categories the primacy effect tended to wear off after a few postshift trials. The results are interpreted as providing an alternative explanation of the frequency effect and the number-of-categories effect discovered by Parducci. The present paper considers the frequency effect a special case of the primacy effect. The explanation is based on the fact that ordinarily stimulus frequency and time of first occurrence are confounded.