Frequency Effects in Distributions of Stimuli with or without Standards

Category ratings of stimulus intensities are affected by the skewing of frequency distributions of stimuli. According to Parducci’s theory, frequency effects are due to the tendency of participants to use all the categories with the same frequency. According to Haubensak’s theory, these effects are due to the tendency of participants to use all categories during the initial presentations. Frequency effects should be reduced by the presentation of stimuli with extreme intensities as standards before each variable stimulus. In this experiment, participants rated brightnesses of squares or lengths of lines using integers from 1 to 5. Target stimuli were presented with or without standards and, for each of these two conditions, frequency distributions were negatively or positively skewed. Results show that the skewing of frequency distributions strongly affected subjective scales independently of the presence or absence of standards. This finding indicates that, in spite of the standards, participants used previous stimuli rather than the standards as a frame of reference for their judgments. Absolute subjective judgments of stimulus intensities are affected by the way stimuli are presented to participants (Garner, 1954; Parducci & Marshall, 1961; Poulton, 1979). When participants rate perceptual magnitude produced by stimuli, the final subjective scales are biased if some of the stimuli have a greater frequency of occurrence. Parducci (1963) proposed a theory about the effects of skewed distributions of stimuli on subjective scales. According to this theory, when a participant rates the magnitude of a perceptual attribute, his judgment is ruled by two principles of judgment. The first principle, called the range principle, is the assignment of categories to each stimulus by dividing the range of stimuli into a number of equal subranges corresponding to the number of available categories. The second principle, called the frequency principle, is the assignment of categories in such a way that each category is used with the same frequency. In his experiments about frequency effect, Parducci (1982; Parducci & Perrett, 1971; Parducci & Wedell, 1986) had participants preview a set of stimuli selected so to be representative of the global range of stimuli used during the regular experimental sessions. That is, when the frequencies of stimuli were varied, also the preview stimuli were varied so to be as representative as possible of the regular series. This procedure could have facilitated participants to judge initial stimuli, because they could have used the stimuli of the preview as standards. It is also possible that participants forgot these standards during the experiment, and then began using the preceding stimuli of the regular series as a frame of reference, producing, as a consequence, biased subjective scales. Haubensak (1990, 1992) proposed another theory of frequency effects on category ratings. Because more frequent stimuli more probably appear in the initial part of the stimulus series, participants center their scale on the restricted range of the most frequent stimuli, without changing it when, subsequently, the less frequent stimuli appear. Plausibly, this happens because participants do not know the whole range of stimuli and, therefore, they use up all categories during the initial presentations of stimuli, especially when the number of categories is small.