A class of multidimensional latent class irt models for ordinal polytomous item responses

We propose a structural equation model, which reduces to a multidimensional latent class item response theory model, for the analysis of binary item responses with non-ignorable missingness. The missingness mechanism is driven by two sets of latent variables: one describing the propensity to respond and the other referred to the abilities measured by the test items. These latent variables are assumed to have a discrete distribution, so as to reduce the number of parametric assumptions regarding the latent structure of the model. Individual covariates may also be included through a multinomial logistic parametrization of the probabilities of each support point of the distribution of the latent variables. Given the discrete nature of this distribution, the proposed model is efficiently estimated by the Expectation-Maximization algorithm. A simulation study is performed to evaluate the finite-sample properties of the parameter estimates. Moreover, an application is illustrated to data coming from a Students ’ Entry Test for the admission to some university courses