Policymakers tend to emphasize differences in the teachers rather than differences in the students at low-performing schools, with ‘low-performing’ typically defined on the basis of average test scores. Because many teacher characteristics (e.g., certification status, educational attainment) do not clearly associate with student achievement, there is an increased reliance on differences in student achievement gains themselves as an indicator of differences in teacher quality. Houston Independent School District (HISD) contracts with EVAAS (Education Value-Added Assessment System) to obtain individual value-added scores for all of its core teachers of grades 3-8 (core referencing teachers who teach Language Arts, Reading, Math, Science and/or Social Studies). EVAAS, the most widely implemented valueadded methodology across the nation, ostensibly isolates the effect of the teacher from all other factors that influence student achievement gains. Critics argue value-added methodology cannot sufficiently account for the nonrandom sorting of teachers across schools and students. EVAAS does not provide transparent or specific descriptions of their methodology, and there has been very little external statistical evaluation of EVAAS scores. An earlier study from the Houston Education Research Consortium (HERC) showed teachers in HISD’s highest need schools received lower valueadded scores on average than teachers in HISD’s lower need schools. It was unclear whether this was an indication that HISD’s highest need schools staffed lower quality teachers or whether teachers at the highest need schools were being penalized because of issues with EVAAS’ methodology. This study used longitudinal data on teachers in the HISD between 2007-08 and 2012-13, as well as data on their students and schools, to investigate the degree to which value-added scores capture the impact of teachers on student achievement gains rather than the influence of factors outside of teachers’ control.