This report describes results from field-testing of POWERSOURCE[C] formative assessment alongside professional development and instructional resources. The researchers at the National Center for Research, on Evaluation, Standards, & Student Testing (CRESST) employed a randomized, controlled design to address the following question: Does the use of POWERSOURCE[C] strategies improve 6th-grade student performance on assessments of the key mathematical ideas relative to the performance of a comparison group? Sixth-grade teachers were recruited from 7 districts and 25 middle schools. A total of 49 POWERSOURCE[C] and 36 comparison group teachers and their students (2,338 POWERSOURCE[C], 1,753 comparison group students) were included in the study analyses. All students took a pretest of prerequisite knowledge and a transfer measure of tasks drawn from international tests at the end of the study year. Students in the POWERSOURCE[C] group used sets of formative assessment tasks. POWERSOURCE[C] teachers had exposure to professional development and instructional resources. Results indicated that students with higher pretest scores tended to benefit more from the treatment as compared to students with lower pretest scores. In addition, students in the POWERSOURCE[C] group significantly outperformed control group students on distributive property items and the effect was larger as pretest scores increased. Results, limitations and future directions are discussed. Four appendices are included: (1) Item Analysis Results of POWERSOURCE[C] pretest; (2) POWERSOURCE[C] Pretest questionnaire; (3) POWERSOURCE[C] Pretest questionnaire; and (4) Descriptive Statistics of Posttest scores by District and Treatment, School, Teacher in Between-School Design and Teacher in within-School (W-S) Design. (Contains 12 figures and 29 tables.)