A Probabilistic IRT Model for Unfolding Preference Data

A probabilistic model is developed for the pair- comparison design in which the unfolding principle that governs the choice process uses a discriminal pro cess analogous to Thurstone's Law of Comparative Judgment. However, this process is governed by the square of the distance between the location of the per son and the stimulus, rather than controlled by the lo cation of the stimulus as in Thurstone's formulation. A simulation study demonstrates the feasibility of esti mation, and two examples use real data to show the implications of the unfolding models for psychological research. Index terms: choice data, item response theory, pair comparisons, preference data, unfolding.