Huanping Dai
Psychoacoust. Lab., Dept. of Psychol., Univ. of Florida, Gainesville, FL 32611-2065
Niek J. Versfeld
TNO Inst. for Human Factors, 3769 ZG Soesterberg, The Netherlands
David M. Green
Univ. of Florida, Gainesville, FL 32611
In the same-different task there are two stimulus alternatives, and the decision maker must decide whether they are the same or different. Two likelihood-ratio decision rules are derived by treating the stimuli as Gaussian random variables with means that depend on the stimulus conditions. They depend on the correlation between the observations. One decision rule is optimum when the observations are independent. This corresponds to experiments in which the standard is fixed. The other rule is optimum when the observations are highly correlated. Such a high correlation is likely to occur when the standard is randomly varied over trials. A COSS (conditional on a single stimulus) analysis provides a convenient way of estimating the rule actually used by a decision maker in a typical decision task. [Work supported by the U. S. AFOSR and the Dutch Ministry of Defence.]