ABSTRACT
Mokken (1971) titled his book A Theory and Procedure of Scale Analysis. He proposed two models that were based on assumptions that nowadays locate them in the family of item response theory (IRT) models. The novelty of Mokken's approach was that item response functions were not parametrically defined but were subjected only to order restrictions. For example, each item response function is assumed to be nondecreasing in the latent variable. The use of only order restrictions led later authors to speak of nonparametric IRT models (Sijtsma and Molenaar, 2002). The Mokken models are less restrictive than several of the alternative IRT models based on parametric response functions, such as the one-, two-, and three-parameter logistic models and their normal-ogive analogs.
