A Latent Class IRT Approach to Defining and Measuring Language Proficiency

English language learner (EL) status has high stakes implications for determining when and how ELs should be evaluated for academic achievement. In the US, students designated as English learners are assessed annually for English language proficiency (ELP), a complex construct whose conceptualization has evolved in recent years to reflect more precisely the language demands of content area achievement as reflected in the standards of individual states and state language assessment consortia, such as WIDA and ELPA21. The goal of this paper was to examine the possible role for and utility of using content area assessments to validate language proficiency mastery criteria. Specifically, we applied mixture item response models to identify two classes of EL students: (1) ELs for whom English language arts and math achievement test items have similar difficulty and discrimination parameters as they do for non-ELs and (2) ELs for whom the test items function differently. We used latent class IRT methods to identify the two groups of ELs and to evaluate the effects of different subscales of ELP (reading, writing, listening, and speaking) on group membership. Only reading and writing were significant predictors of class membership. Cut-scores based on summary scores of ELP were imperfect predictors of class membership and indicated the need for finer differentiation within the top proficiency category. This study demonstrates the importance of linking definitions of ELP to the context for which ELP is used and suggests the possible value of psychometric analyses when language proficiency standards are linked to the language requirements for content area achievement.