1 Teachers College Record Volume 117, 040308, April 2015, 40 pages Copyright © by Teachers College, Columbia University 0161-4681 Using Data to Alter Instructional Practice: The Mediating Role of Coaches and Professional Learning Communities JULIE A. MARSH University of Southern California MELANIE BERTRAND Arizona State University ALICE HUGUET University of Southern California Background: Despite increased access to student learning data, scholars have demonstrated that teachers do not always know how to use these data in ways that lead to deep changes in instruction and often lack skills and knowledge to interpret results and develop solutions. In response, administrators have invested in instructional coaches, data coaches, and profes- sional learning communities (PLCs) to support teachers in this process. Despite their popular- ity, there is limited research on the ways in which coaches and PLCs mediate teachers’ use of data and the various types of expertise brought to bear on this process. Purpose: This exploratory study examined how working with a coach or PLC shaped teach- ers’ responses to data in six middle schools and the factors that influenced the activities and effects of coaches and PLCs. Our intent was to deeply examine processes and identify key constructs and relationships to guide future research and practice. Research Design: Our research involved a year-long comparative case study of six low-per- forming middle schools in four districts that supported teacher data use via literacy coaches, data coaches, or PLCs. We draw on cultural historical activity theory and data from 92 interviews, 6 focus groups, 20 observations of meetings, and monthly surveys of case study teachers (15), coaches (4), and PLC lead teachers (2). Findings: We found that coaches and PLCs played important roles in mediating teachers’ responses to data and were often associated with instances in which teachers used data to alter their instructional delivery (as opposed to surface-level changes in materials and top- ics). Further, the dynamic relationship between vertical expertise (an individual’s knowledge and skills) and horizontal expertise (knowledge that is co-created through interactions and movement across contexts) may help explain the ways in which PLCs and coaches facilitated deeper level changes in pedagogy. Finally, dialogue was a central mediating practice, and school leadership and the district-level context shaped the possibility for change.