THURSTONIAN MODELS FOR VARIANTS OF THE METHOD OF TETRADS

Variants of the method of tetrads are discussed and their psychometric functions are both derived and presented in table form. For the first variant, the Unspecified method, subject response is correct provided that the subject, when presented with two stimuli from one set and tw o stimuli from a I;second set, correctly segregates the four stimuli i nto their two appropriate groups. in the second variant, the Specified method, subject response is correct provided that the subject., when presented with two stimuli from one set and two stimuli from a second set, correctly discerns the two strongest stimuli. For the Unspecified method, P-c=1 - 2 integral(-infinity)(infinity) phi(x)[2 Phi(x)Phi(x- delta)-[Phi(x-delta)](2)]dx, where P-c is the probability of correct r esponse, and phi(x) and Phi(x) are the density and distribution functi ons respectively, for the standard normal distribution. The parameter delta is the perceptual difference between stimulus means and correspo nds to the signal detection measure of sensitivity d'. For the Specifi ed method, P-c=1-2 integral(-infinity)(infinity)phi(x)Phi(x)[2 Phi(x-d elta)-[Phi(x-delta)](2)]dx For each of the two variants, tables connec t the probability of correct response to delta.