As the debate over releasing teacher performance data continues, many in the anti-data dump camp (read: unions and friends) have settled on an argument that can quickly and somewhat unfairly be characterized as: "It's not our fault!"
Here's the longer version: Value-added data, which compares students' scores at the beginning of the year and the end of the year for each teacher, isn't a fair measure of teacher performance, because student home life, economic resources, language skills, and other factors outside of the classroom have a bigger impact on achievement than anything teachers can do.
Certainly teachers are very important - the most important cause of student success within the school. But scholars agree that factors outside the school are even more important than the teacher, especially family income. Most economists estimate that teachers are responsible for about 10% to 20% of the variation in student scores, and that outside-the-school factors influence about 60% of student learning gains.
The first sentence explains why the argument in the rest of the paragraph is irrelevant to the debate. Teachers may not be the be-all-and-end-all of every student's life, but they are an important variable that should be easily tweakable. Changing the socio-economic status, immigration history, or disability status of most students isn't feasible. There's not much a middle school principal can do about the education attainment of a student's parents or the amount of time they have to devote to supervising homework either. But middle school principals do (or could) have the power to hire and fire teachers.
Ravitch notes that the data New York wants to release would be based on a single year's gains, unlike the Los Angeles Times data dump, which spanned 7 years. Fair enough, any teacher can get a crap batch of kids for one year. But consistent failure to add value across many years and many groups of kids—especially within schools where other teachers are producing bigger gains with kids who are demographically identical—is a sign that something is wrong with the teacher. A brilliant teacher can't fix everything, but that doesn't mean bad teachers shouldn't be exposed and fired.