Investigation of within group heterogeneity in measurement comparability research

This dissertation investigated the broad concern of the violation of the within group homogeneity (WGH) assumption associated with methods used to investigate differential item functioning (DIF) in educational large-scale assessments (ELSAs) in two studies. The first study investigated the WGH assumption using real data from an international ELSA of reading literacy administered to examinees from four countries: Chinese Taipei, Hong Kong, Qatar and Kuwait. It examined the degree of homogeneity of the test data structure, and the consistency in identification of DIF items between a manifest versus a latent class DIF approach. Sources of incomparability underlying DIF items related to reading comprehension processes, cognitive complexity of items and purposes of reading as well as instruction and teacher-related variables were investigated. Three latent classes (LCs) were identified indicating heterogeneity in response patterns. The two approaches identified different DIF items with the latent class approach identifying a higher number of differentially functioning items with larger effect sizes. Inconclusive results were obtained when analyzing sources of manifest DIF items. Student achievement was found to be a primary factor influencing LC membership, whereas typically used manifest characteristics (e.g., country or gender) had either small or no significant influence on LC assignment. The second study used simulated data to systematically investigate the effects of heterogeneity on accuracy of DIF detection by two manifest DIF detection methods: logistic regression and Mantel-Haenszel. Greater degrees of heterogeneity resulted in significantly reduced DIF detection accuracy. Findings from this dissertation have important implications for DIF and validity research. The first study presents evidence of within group heterogeneity in international assessments and therefore, contributes to research that suggests that the WGH assumption needs to be tested for meaningful manifest DIF analysis. Moreover, findings from study one indicate that measurement incomparability may be greater among groups that are not defined by manifest variables such as gender, language and ethnicity. Findings from study two highlight the importance of examining within group heterogeneity in measurement comparability analyses; otherwise, violations of the WGH assumption may lead to grossly overestimating comparability claims.Education, Faculty ofEducational and Counselling Psychology, and Special Education (ECPS), Department ofGraduat