A procedure for detecting variability of psychophysical thresholds

In adaptive psychophysical testing, it is desirable to identify experimental runs during which the subject's threshold changes. This is not easy to do, because the testing level changes during the course of the run. The following screening procedure can assist in detecting threshold variability. At the conclusion of the run, make a maximum‐likelihood estimate of the psychometric function. That is, for some assumed parametric form of the psychometric function, determine values of the parameters that maximize the probability of the observed set of responses. Next, for each trial determine the value of the random variable Z = Y − Px(C). Px(C) is the probability of a correct response at testing level x, given by the fitted psychometric function, and Y is a random variable equal to 1 for a correct response and equal to 0 for an incorrect response. Finally, determine the slope of the linear regression of Z upon trial number. If the subject's response behavior is stable, then E(Z) = 0 regardless of testing level...