Evaluation of the Bifactor Nominal Response Model Analysis of a Health Efficacy Measure.

The bifactor nominal response item response theory (IRT) model, proposed by Cai, Yang and Hansen (2011), provides an extension of Bock's (1972, 1997) unidimensional nominal response model to multidimensional IRT. This model has not been utilized in any published studies since its original development. In this study, the model was applied to data from a sample of college students (N = 799) to evaluate the psychometric properties of a health efficacy measure. The nominal response model has the unique capability to estimate the functioning of each single response category, and higher response categories were found to have better functioning in this study. Poor-functioning categories were identified and combined into their adjacent categories. Items with revised response format showed improved functioning. The bifactor nominal response model is a useful tool for evaluation of bifactor scales with ordered while non-equivalently functioning categories.