How do you determine teacher effectiveness?

How do you determine teacher effectiveness?
Sent to USA Today, Jan. 26, 2012

“States weaken tenure rights for teachers” (Jan. 25) emphasizes the importance of evaluating teacher effectiveness.
A major problem is that these evaluations are often based on students gains on standardized tests, called “value-added” measures.
A number of studies have shown that value-added measures are very unstable: Teachers' ratings based on previous years are weak predictors of test scores at the end of a year with new students. A teacher who succeeds in boosting scores with one group will not necessarily succeed with others. Different tests can result in different scores for the same teacher.
Value-added evaluations also ignore the huge impact of factors beyond the teachers’ control. Finally, there are ways of pumping up test scores without student learning, including teaching test-taking strategies and making sure weak students don't take the test.
Nobody objects to teachers being evaluated on their effectiveness. Using gains on standardized tests is a bad way to do it.

Stephen Krashen
Professor Emeritus, University of Southern California

Sources:
Not stable: Sass, T. 2008. The stability of value-added measures of teacher quality and implications for teacher compensation policy. Washington DC: CALDER. (National Center for Analysis of Longitudinal Data in Educational Research.) Kane, T. and Staiger, D. 2009. Estimating Teacher Impacts on Student Achievement: An Experimental Evaluation. NBER Working Paper No. 14607 http://www.nber.org/papers/w14607;
Different tests result in different value-added scores: Papay, J. 2010. Different tests, different answers: The stability of teacher value-added estimates across outcome measures. American Educational Research Journal 47,2.

Original article:
http://www.usatoday.com/news/education/story/2012-01-25/teacher-tenure-rights-firings/