Working Memory Impairments in Schizophrenia Patients: A Bayesian Bivariate IRT Analysis

Several studies have shown that spatial working memory is impaired in schizophrenia patients. In our study, schizophrenia patients and normal controls participated in a memory test designed to measure both spatial and object working memory. Standard analyses were inappropriate because the test items had differing levels of difficulty. To account for this problem, the data were analyzed using a Bayesian Bivariate Item Response Theory (BBIRT) model. Item response theory is a method for analyzing test scores in which the test items themselves are parameterized in addition to the test-takers’ abilities. Analyzing the data in this way accounts for the fact that the questions were not all equally difficult, and also produces results which are more generalizable and less test-dependent. The analysis was carried out using Gibbs sampling, a Markov Chain Monte Carlo technique which improves upon standard EM methods for IRT models by producing standard error estimates which more accurately represent uncertainty about the parameters.