A long article in the NYT that makes a very fair point: that value-added systems are far from perfect and must be improved, and also need to be used with care when evaluating teachers.  But we mustn't let perfection be the enemy of the good.  Even if the system can only correctly identify the very worst teachers, that's REALLY valuable given the damage such teachers do to children:

But the experience in New York City shows just how difficult it can be to come up with a system that gains acceptance as being fair and accurate. The rankings are based on an algorithm that few other than statisticians can understand, and on tests that the state has said were too narrow and predictable. Most teachers' scores fall somewhere in a wide range, with perfection statistically impossible. And the system has also suffered from the everyday problems inherent in managing busy urban schools, like the challenge of using old files and computer databases to ensure that the right teachers are matched to the right students.

All of this was not as important when the teacher rankings were an internal matter that principals could choose to heed or ignore. City officials had pledged to the teachers' union that the rankings would not be used in the evaluation of teachers and that they would resist releasing them to the public.

But over the past several months, the system of teacher rankings has been catapulted to one of the most contentious issues facing the city's 80,000-member teaching force. A new state law, passed this year to help New York win Race to the Top money, pledges that by 2013, 25 percent of a teacher's evaluation be based on a value-added system. The city has begun urging principals to consider rankings when deciding whether to grant tenure. And the city now supports the release of the data to the 12 media organizations, including The New York Times, that have requested it.

The departing schools chancellor, Joel I. Klein, defended the release of the rankings in an e-mail to school staff members, acknowledging that they had limitations but calling them "the fairest systemwide way we have to assess the real impact of teachers on student learning."

"For too long," Mr. Klein wrote, "parents have been left out of the equation, left to pray each year that the teacher greeting their children on the first day of school is truly great, but with no real knowledge of whether that is the case, and with no recourse if it's not."

But the United Federation of Teachers, the city's teachers' union, has sued to keep names in the rankings private, arguing that the data is flawed and would result in unnecessary harm to the reputation of teachers. The matter is now before Justice Cynthia Kern of State Supreme Court in Manhattan.

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Hurdles Emerge in Rising Effort to Rate Teachers

By SHARON OTTERMAN

Published: December 26, 2010

www.nytimes.com/2010/12/27/nyregion/27teachers.html