In this study, we investigated differential item functioning (DIF) and its sources using a latent class (LC) modeling approach. Potential sources of LC DIF related to instruction and teacher-related variables were investigated using substantive and three statistical approaches: descriptive discriminant function, multinomial logistic regression, and multilevel multinomial logistic regression analyses. Results revealed that differential response patterns, as indicated by identification of LCs, were most strongly associated with student achievement levels and teacher-related variables rather than manifest characteristics such as gender, test language, and country, which are the focus of typical measurement comparability research. Findings from this study have important implications for measurement comparability and validity research. Evidence of within-group heterogeneity in the test data structure suggests that the identification of DIF and its sources may not apply to all examinees in the group and that measurement incomparability may be greater among groups that are not defined by manifest variables such as gender and ethnicity. Results suggest that alternative variables that may be more closely related to the investigated construct should be examined when conducting measurement comparability research. (Contains 4 tables and 1 footnote.)