A Thurstonian model for the dual pair (4IAX) discrimination method.

In the dual pair method, the subject is presented with two stimuli in two pairs: One pair is composed of two samples of the same stimulus; the other pair is composed of two samples of different stimuli, one being the same as that in the identical pair. The task of the judge is to select the most different pair. The psychometric function for the dual pair method is derived and expressed in terms of a singly noncentral beta distribution. A table is provided that connects a measure of the degree of difference, d, to the probability of a correct response. This table assumes an unbiased observer and differencing decision rule. A table is provided to give an estimate of the variance of d , the experimental estimate of d. The power of the dual pair method is also investigated, and a formula to determine the sample size required to meet Type I and Type II error specifications is given. The dual pair method appears to be slightly less powerful than the duo-trio and the triangular methods. Experimental investigation is needed to explore the dual pair in applied research work.