The Carnegie Knowledge Network have an interesting brief looking at what we know about value-added variation among educators who teach at different schools, the correctly note that
Some schools are more effective than others by virtue of their favorable resources, leadership, or organization; we can expect that teachers of similar skill will perform better in these more effective schools.
This then adds a new wrinkle to using value-added to evaluate the efficacy of individual teachers. Not only can value-added scores be dramatically affected by the socioeconomic conditions in the community and student body, but significant variation can exist between schools in the same communities. Carnegie notes
This brief has considered sources of potential bias when we use value-added scores to compare teachers working in different schools. A growing body of evidence suggests that schools can vary substantially in their effectiveness, potentially inflating the value-added scores of teachers assigned to effective schools. Schools also vary in contextual conditions such as parental expectations, neighborhood safety, and peer influences that may directly support learning or that may contribute to school and teacher effectiveness. Moreover, schools vary substantially in the backgrounds of the students they serve, and conventional statistical methods tend to break down when we compare teachers serving very different subsets of students.
Carnegie recommends much more study and analysis, and concludes that the possible stratification of teachers into sub-groups serving similar students, complicate value-added analysis and may not be congruent with policymakers’ wish to compare all teachers in a district.