On Wilcox's latent structure model for guessing

Wilcox (1979) has presented in this journal a model for achievement testing where a population of examinees is described in terms of a domain of skills. In most of his paper each examinee knows a proportion ζ of all items from this domain, and has a probability β of guessing the correct response given that he/she does not know it. Across examinees, the parameters ζ and β (1 — ζ) have a bivariate Dirichlet distribution. Per examinee, ζ and β are estimated from the results on pairwise equivalent item pairs. The present note explores the feasibility of the assumptions and the usefulness of the results of Wilcox's paper. It thus attempts to show the obstacles between the present psychometric model and a fully satisfactory treatment of guessing.