This publication is one of three policy briefs designed to guide local policymakers in undertaking justice reinvestment, a data-driven strategy to identify the drivers of criminal justice system costs and make more efficient use of resources while maintaining public safety. Using the fictional example of Doe County, the brief details the data that various agencies must share and analyze in order to identify cost and population drivers, and provides examples of how data analysis findings can be used to formulate policy changes and reinvestment strategies. Additional resources and a "getting started" worksheet are included in an appendix.