Extending an Identified Four-Parameter IRT Model: The Confirmatory Set-4PNO Model

Given the frequent presence of slipping and guessing in item responses, models for the inclusion of their effects are highly important. Unfortunately, the most common model for their inclusion, the four-parameter item response theory model, potentially has severe deficiencies related to its possible unidentifiability. With this issue in mind, the dyad-four-parameter normal ogive (4PNO) model was developed. This model allows for slipping and guessing effects by including binary augmented variables-each indicated by two items whose probabilities are determined by slipping and guessing parameters-which are subsequently related to a continuous latent trait through a two-parameter model. Furthermore, the dyad-4PNO assumes uncertainty as to which items are paired on each augmented variable. In this way, the model is inherently exploratory. In the current article, the new model, called the set-4PNO model, is an extension of the dyad-4PNO in two ways. First, the new model allows for more than two items per augmented variable. Second, these item sets are assumed to be fixed, that is, the model is confirmatory. This article discusses this extension and introduces a Gibbs sampling algorithm to estimate the model. A Monte Carlo simulation study shows the efficacy of the algorithm at estimating the model parameters. A real data example shows that this extension may be viable in practice, with the data fitting a more general set-4PNO model (i.e., more than two items per augmented variable) better than the dyad-4PNO, 2PNO, 3PNO, and 4PNO models.