This document is a supplement to the Medicaid 1115 Demonstration Evaluation Design Plan prepared by Mathematica Policy Research and submitted to the Centers for Medicare & Medicaid Services (CMS) in May 2015 (Irvin et al 2015). The supplement presents our approach to the interim evaluation of the section 1115 Delivery System Reform Incentive Payment (DSRIP) demonstrations. The purpose of the interim evaluation, which we will conduct in 2017, is to examine the preliminary effects of the DSRIP demonstrations on delivery system transformation and on specific clinical quality outcomes. The results of this evaluation will provide useful information about the short-term effects of the demonstrations on key outcomes and on the settings in which the DSRIP has been most effective. The final evaluation in 2019 will assess the more mature impacts of the demonstration on provider readiness for value-based purchasing, on population health, and on the total cost of care.