Interim tests are a central component of district-wide assessment systems, yet their technical quality to guide decisions (e.g., instructional) has been repeatedly questioned. In response, the study purpose was to investigate the validity of a series of English Language Arts (ELA) interim assessments in terms of dimensionality and prediction of summative test performance, based on Grade 6 student data (N = 4,651) from a larger, urban district. Factor analytic results supported modeling the interim test data in terms of a bifactor model (Gibbons & Hedeker, 1992), with items reporting moderate to high relationships to the primary dimension (i.e., ELA) and varying estimates on the secondary domains. Hierarchical multiple linear regression results indicated that primary ELA scores were the strongest predictors of summative test performance, with subscale scores not improving predictive accuracy. Findings address issues pertaining to investigating the technical quality of test data widely used in district-wide assessment systems.