Optimal stimulus placement in adaptive psychophysical procedures
David Michael Green · The Journal of the Acoustical Society of America · 1989
A common misconception is that the stimulus should be presented near the threshold value in an adaptive psychophysical procedure to minimize the variance of threshold estimates. For maximum likelihood procedures, recent work by Laming, Watson, and Pelli shows that the stimulus should be presented at a relatively high level. For some psychometric functions, they found the optimal stimulus placement level produces 84% to 94% correct in a two-alternative forced-choice task. This result is particularly disquieting, because the popular two-down one-up rule tracks a relatively low percentage of correct responses, 70.7%. Our computer simulations, using a variety of psychometric functions, confirm the validity of their analysis. These simulations also demonstrate that the precise form of the psychometric function is not critical in achieving high efficiencies. Finally, data from human listeners indicate that the standard deviation of threshold estimates is approximately twice as large when the stimulus presented on each trial is at a stimulus level corresponding to 70.7%, rather than 94% correct. [This research was supported by NSF and AFOSR.]