This mixed design study investigated the predictive and instructional uses of two different types of interim mathematics assessments given in two different districts. One district administered the same summative type of assessment three times a year, while the other district administered a different interim assessment after six-week intervals of instruction. The quantitative component consisted of conducting several multiple regression analyses of students' mathematics results for grades four through ten on three different district benchmark assessments to identify if there were differences in the strengths of the relationships between student performance on the interim assessments and on their performance on the California Standards Tests for mathematics. For the qualitative component, analysis was conducted on data from teacher collaboration meeting observations and district and site interviews of stakeholders that shed light on those practices that best supported modification of instruction and improvement of teaching and learning. Quantitative results suggested that the overall student performance on both types of interim assessments used by District A and B respectively, had significant and strong positive correlations to student performance on the California State Tests California State Tests for grades 4-6 and results for grades 7-10 had significant and moderate, positive correlations to student performance on CSTs. Qualitative results identified conversations and behaviors that promoted or extended current data-informed decision-making theory. Five areas surfaced as contributing to teachers' capacity to make data actionable that relate to phases of the Data-Informed Decision Making Framework. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]