Stimulus step size and heterogeneous stimulus conditions in adaptive psychophysics

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This paper reports on simulations of repeated estimates of stimulus thresholds using a two-down, one-up adaptive procedure and forced-choice procedures using 2, 3, and 4 response alternatives. In the first section of the paper, the selection of an initial and final step size for the stimulus is the main focus. For a 2-alternative forced-choice psychometric function, a step size of roughly one-quarter the useful range (60%–90%) provides a reasonable initial step size. A final step size of roughly half that value is recommended. The increase in the variability of threshold estimates is, however, not very great if values near those recommended are selected instead. The change in the variability of the threshold estimates with the change in the number of alternatives used in the forced-choice procedure can be predicted by considering the sweat factor associated with the psychometric function. In the second part of the paper, the focus is on how the variability in threshold estimates increases if the stimulus set is heterogeneous; that is, the stimuli produce different psychometric functions. Stimuli that differ only in their means increase the variability of threshold estimates more than do stimuli that differ only in the slopes of their psychometric functions. Stimuli that contain both forms of heterogeneity, however, increase the variability in threshold estimates by only about 50%.