In this study, the authors connect 3 streams of literature to develop an instrument for measuring the degree to which principals engage in data-informed decision making on high-impact strategies that are empirically associated with higher student achievement. The 3 literature streams are (a) the importance of data-informed decision making, (b) the role of principals in school effectiveness, and (c) high-impact strategies for raising student achievement. The authors used Marzano's (2003) 11 high-impact strategies as a framework to develop items and collected data from 256 principals in Michigan to test the reliability (internal consistency) and (construct and factorial) validities of the instrument. The instrument, "Data-Informed Decision-Making on High-Impact Strategies: An Inventory for Principals," demonstrates good psychometric properties. Theoretical and practical implications are discussed. (Contains 6 notes, 5 tables, and 4 figures.)