Prominent testing expert Robert Linn concluded in his workshop paper: “As with any effort to isolate causal effects from observational data when random assignment is not feasible, there are reasons to question the ability of value-added methods to achieve the goal of determining the value added by a particular teacher, school, or educational program” (Linn, 2008, p. 3). Teachers are not assigned randomly to schools, and students are not assigned randomly to teachers. Without a way to account for important unobservable differences across students, VAM techniques fail to control fully for those differences and are therefore unable to provide objective comparisons between teachers who work with different populations. As a result, value-added scores that are attributed to a teacher or principal may be affected by other factors, such as student motivation and parental support.
Will Arne Duncan Read This?
Ken Libby over at Our Global Education has a find; the National Academy of Sciences has a report that pretty much flies in the face of the Obama/Duncan education reform trajectory. A snippet: