Research on mass incarceration usually centers on state-level data: state prison populations or the statewide combined prison and jail population. Using the state as the unit of analysis is sufficient for understanding the broad contours of incarceration in the United States, but it does not provide the level of detail necessary to help policymakers reduce the use of incarceration. This is because although state law dictates criminal penalties, these laws are interpreted and deployed in practice by police, prosecutors, judges, and others at the local level (counties and cities). Therefore, county-level data, and specifically data on how counties use the local jail, are fundamental to a thorough understanding of the justice system.