In this report, RAND researchers investigate one specific issue that may contaminate utilization of COVID-19--era school-aggregate scores and result in faulty comparisons with historical and other proximal aggregate scores: changes in school composition over time. To investigate this issue, they examine data from NWEA's Measures of Academic Progress (MAP) Growth assessments, interim assessments used by states and districts during the 2020-2021 school year. This report has four main sections. First, it provides background information on the assessment context. A detail is provided on the problems caused by changes in test-taking populations, particularly for school-level analyses, and provides examples of state policies for score reporting. Second, it details the research questions and describes the sample and methods. Third, it presents the results of the investigations. The report concludes with a discussion of the implications of findings for decision-making during the COVID-19 crisis. This report is the second of three that examine the impacts of COVID-19-related assessment disruptions on school and district processes. The first report, "Adapting Course Placement Processes in Response to COVID-19 Disruptions: Guidance for Schools and Districts. Research Report. RR-A1037-1" (ED612136), compares three strategies to estimate missing test scores and help with course placement decisions.