THE DIFFERENCE MODEL WITH GUESSING EXPLAINS INTERVAL BIAS IN TWO-ALTERNATIVE FORCED-CHOICE DETECTION PROCEDURES

The standard difference model of two-alternative forced-choice (2AFC) tasks implies that performance should be the same when the target is presented in the first or the second interval. Empirical data often show interval bias in that percentage correct differs significantly when the signal is presented in the first or the second interval. We present an extension of the standard difference model that accounts for interval bias by incorporating an indifference zone around the null value of the decision variable. Analytical predictions are derived which reveal how interval bias may occur when data generated by the guessing model are analyzed as prescribed by the standard difference model. Parameter estimation methods and goodness-of-fit testing approaches for the guessing model are also developed and presented. A simulation study is included whose results show that the parameters of the guessing model can be estimated accurately. Finally, the guessing model is tested empirically in a 2AFC detection procedure in which guesses were explicitly recorded. The results support the guessing model and indicate that interval bias is not observed when guesses are separated out. PRACTICAL APPLICATIONS This research provides practicing sensory scientists with an alternative approach to obtaining unbiased sensitivity measures from two-alternative forced-choice detection or discrimination tasks in which a no-preference response has been allowed and placebo trials are included (i.e., trials in which both stimuli are identical). Experiments designed in this way imply what can naturally be referred to as a two-alternative non-forced-choice task. Analysis of the resultant data under the difference model with guessing allows obtaining an unbiased measure of sensitivity as well as an estimate of the size of the indifference zone, whether in detection or in discrimination tasks. In the latter, this indifference zone explains why a non-negligible percentage of preference responses are given in placebo trials during paired discrimination tests and, through the guessing model, it also accounts for a non-negligible number of nonidentical calls to actually identical stimuli in same-different tests.