eng TQMP Tutorials in Quantitative Methods for Psychology 1913-4126 2010-09-01 6 2 39 51 article Marlène Galdin marlene_galdin@ ssss.gouv.qc.ca 1 Louis Laurencelle louis.laurencelle@uqtr.ca 2 Centre de réadaptation en déficience intellectuelle et troubles envahissants du développement Mauricie/Centre-du-Québec - Institut universitaire Université du Québec à Trois-Rivières Statistical properties of the ability level estimate (theta) in item response theory (IRT) were investigated through a Monte Carlo investigation, based on data generated with a four cases multifactor design. Dichotomous items and the logistic two-parameter IRT model in a one-dimensional setting have been chosen. The estimation procedure was the marginalized Bayesian item parameters estimation and EAP estimation for theta. The property of invariance is discussed. Results show that estimation of theta is intrinsically biased, is constrained by the number of items and that it performs better when the number of items and the number of examinees increase. Furthermore, IRT parameters do not seem to perform better nor give more information than those used in classical test theory. http://www.tqmp.org/Content/vol06-2/p039/p039.pdf Item response theory Ability parameter Estimation Invariance, Monte Carlo