Re: Psychophysical measurement technique - PEST? (at)

Subject: Re: Psychophysical measurement technique - PEST?
From:    at <spfHOQAA.ATT.COM>
Date:    Mon, 1 Mar 1993 09:24:00 EST

Peter, I've generally used a modified up/down procedure, wherein the direction of adjustment is changed based on the correctness of the responses. This is, as I recall, comparable to the gist of PEST. Anyway, the algorithm is based on balancing the number of correct and incorrect responses to get the desired decision level. For two-way forced choice designs, the first and second stimulus sets were about equally likely to contain the signal, the other being noise. If the subject made two correct choices consecutively, for a condition at a given correlation level (the variable I'm usually measuring), the correlation level was reduced by 0.1 for the next trial of that condition. If an error was made, the level was increased by 0.1 for the next trial. Reversals in the direction of correlation level modification occurred at various levels, with the procedure terminating after seven reversals. The first reversal was ignored, and the average of the levels at which the remaining six reversals occurred was taken as an estimate of the threshold of pattern detection. Since, at the stopping point, the probability of being correct twice on a pair of trials equals the probability of not being correct twice on a pair of trials, the probability of being correct on a single trial is equal to the square root of 0.50, or about 0.71. To achieve some other bogey than 0.71, you would adjust the number of trials at each level so that the appropriate root comes out to your bogey. Of course, this will increase the number of trials you need, so if you can live with 0.71 instead of 0.75 you're better off. The reason for increasing the level immediately after receiving an incorrect response, even on the first trial of a pair, is that the second trial offers no information; there is no way to achieve two correct responses. In this way, the number of trials presented to the subject is reduced without effecting the results. Hope this helps. I may actually have some C code lying about which does this, if you're interested. Steve Frysinger

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