Methodology and Application of Item Response Theory: Evaluation of Nonignorable Missingness Approaches and Development of a Speciesism

Item response theory encompasses models and related statistical methods designed to analyze measurement data. Interest in the methodology and application of item response theory has grown in recent years. As the popularity of item response theory increases so does the importance of using simulations to address current methodological challenges. Further, the benefits offered by item response theory call for the application of this approach in the development of new scales. Accordingly, this dissertation is comprised of two discrete studies on item response theory, the first methodological, focused on evaluating nonignorable missingness approaches, and the second applied, focused on developing a speciesism scale. The first study focused on nonignorable item level missingness as this is a commonly encountered problem in item response theory. MIRT models, where the substantive model of interest and the missing data process are modeled simultaneously, have been proposed as a potential solution, but more research is needed to fill existing gaps in the literature. Thus, we evaluated and compared the performance of Bayesian MIRT models in addressing nonignorable missingness in Rasch models using a simulation study. Overall, results showed that the MIRT models produced biased parameter estimates when nonignorable missingness was present and underperformed compared to other methods. The second study focused on improving the measurement of speciesist attitudes. Speciesism involves the oppression and prejudiced treatment of nonhuman animals. Current measurement of speciesist attitudes is hindered by limitations in current scales. To address these limitations, we evaluated a current speciesism scale using a graded response model and used the results to inform the development of novel items. The new scale successfully captured additional speciesism dimensions, but struggled to assess a wider range of the speciesism trait.