Abstract
Probit analysis was applied to the problem of threshold estimation from psychometric functions derived from the two-alternative forced-choice (2AFC) method of constant stimuli. Threshold estimates from 2AFC experiments are surprisingly poor: They are about twice as variable as corresponding estimates based on the traditional yes-no method of constant stimuli, and their asymmetrical confidence limits are not readily predicted from conventional standard error formulas. All of these faults are exacerbated in small samples. Computer simulations demonstrated that, for small samples, the probit analysis equations do not give a valid estimate of threshold variability. The variability of staircase estimates of threshold cannot be less than the variability of threshold estimates derived from the method of constant stimuli given an optimum placement of trials. Hence our findings also define the minimum variability of all staircase estimators under the assumptions of probit analysis.
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