This study examines and compares four person-fit statistics (PFSs) in the framework of the "D"- scoring method (DSM): (a) van der Flier's "U3" statistic; (b) "Ud" statistic, as a modification of "U3" under the DSM; (c) "Zd" statistic, as a modification of the "Z3 (l[subscript z])"…

If questioned about their beliefs, psychometricians in one camp would argue the firm conviction that the Rasch model is mathematically elegant and intuitive as well as plausible for practitioners, pointing out the advantages of a simple model that "counts" every item in the same way. Psychometricians of another camp would argue that the three…

This author has always felt a bit uneasy about the three parameter logistic (3PL) model because it seems about as prone to discovering guessing when guessing does not take place, as the Rasch model is reluctant to admit that guessing might ever take place at all. However, the author has attributed this to the prior "magic" employed when estimating…

This author states that, while the article by Gunter Maris and Timo Bechger ("On Interpreting the Model Parameters for the Three Parameter Logistic Model," this issue) is highly interesting, the interest is not so much in the practical implications, but rather in the issue of the meaning and role of statistical models in psychometrics and…

This paper addresses two problems relating to the interpretability of the model parameters in the three parameter logistic model. First, it is shown that if the values of the discrimination parameters are all the same, the remaining parameters are nonidentifiable in a nontrivial way that involves not only ability and item difficulty, but also the…