Curriculum-based measures (CBMs) such as Word Identification Fluency promote student achievement, but because they are timed and administered frequently, they are prone to variation in student response styles. To study the impact of WIF response styles, we created and examined the validity of a novel response style measure and examined the degree to which it moderated the validity of WIF growth slopes as a predictor of student outcomes. Students who moved quickly through the WIF (speeders) exhibited greater growth compared to non-speeders with similar WIF slopes on a criterion measure, and WIF growth explained the most variance in growth on the criterion measure among speeders. Response style was rather consistent within students and across students within teachers, suggesting that teachers' instructions and testing practices may influence how students approach the tests. Implications of the findings for effective CBM testing are discussed.