Detecting Guessing Effects for the Nominal Categories Model Using a Conditional Likelihood Ratio Test With Data-Dependent Degrees of Freedom

Item response theory models have been extended by including guessing parameters to represent the response tendencies of individuals with low trait levels. Particular cases of these models arise when the guessing parameters are fixed at boundary values. For example, the two-parameter logistic model is derived from the three-parameter logistic model by fixing the guessing parameter to zero. However, the likelihood ratio test statistic for nested pairs of models asymptotically fails to follow a chi-square distribution when some parameters take boundary values, resulting in an unreliable p-value. This article proposes two solutions to this problem: a chi-square distribution with estimated degrees of freedom and a score test to evaluate fit at the item level. Guessing parameters are incorporated into the nominal categories model, and models with and without guessing are compared using the proposed chi-square statistic. The special case of the comparison between the three-parameter logistic and two-parameter logistic models is also considered. A simulation study was conducted to evaluate the performance of the proposed chi-square statistic with dichotomous and polytomous data. The results indicate that it is a viable alternative for detecting significant guessing effects. The solution is further illustrated with an empirical example to demonstrate its practical application.