The use of an implicit standard for measuring discrimination thresholds

We measured thresholds for comparing the separation between lines, using ei ther the method of constant stimuli (MCS) or the method of single stimuli ( MSS). In the MCS an explicit standard is presented on each trial, whereas i n the MSS the standard is the mean of the set. The thresholds for the MSS p rocedure were nearly identical to those with the MCS procedure, whether or not feedback was used. A statistical model is presented showing how the thr eshold error estimated by MSS varies according to the number of past stimul i used by the observer to calculate the mean of the set. If the model is an accurate representation of human processing, our observers were averaging over the last 10-20 trials to estimate the implicit standard. Our results s how that the explicit standard in the MCS procedure is generally superfluou s. Provided that the test range is small, and that the observer is given so me practice trials, thresholds measured with MSS procedure are just as prec ise as those measured with the traditional MCS procedure. (C) 2000 Elsevier Science Ltd. All rights reserved.