The School Composition and the Black-White Achievement Gap study was undertaken by the National Center for Education Statistics to present both descriptive and associative information on the relationships among the percentage of students in a school who were Black (referred to as "Black student density" or "density"), the Black-White student academic performance gap, and student achievement. The study used data from the 2011 Grade 8 mathematics administration of the National Assessment of Educational Progress (NAEP), along with data from the Common Core of Data (CCD). This methodology companion provides details and technical documentation for the two chapters of the report that use statistical analysis, namely those on: (1) the relationship between Black student density and achievement; and (2) the decomposition of the overall gap into between-school and within-school components. [For the full report, see ED560723.]