In the first decade of the 21st century, the New York City (NYC) Department of Education implemented a set of large-scale and much debated high school reforms, which included closing large, low-performing schools, opening new small schools, and extending high school choice to students throughout the district. The school closure process was the most controversial of these efforts. Yet, apart from the general sense that school closures are painful, there has never been a rigorous assessment of their impact in NYC. To begin to fill this gap, the Research Alliance undertook a study of the 29 low-performing high schools that were designated for closure in New York City between 2002 and 2008, looking particularly at the impact of these closures on students' academic performance, attendance, and mobility. Technical appendices in this report include: (1) Supplemental Tables; (2) Sample Definition; and (3) Statistical Models Used to Estimate Impacts of High School Closures. [For the full report, see ED573426. For the brief, see ED573428.]