Same Teachers, Similar Students, Similar Tests, Different Answers
One of my favorite studies to date about VAMs was conducted by John Papay, an economist once at Harvard and now at Brown University. In the study titled “Different Tests, Different Answers: The Stability of Teacher Value-Added Estimates Across Outcome Measures” published in 2009 by the 3rd best and most reputable peer-reviewed journal, American Educational Research Journal, Papay presents evidence that different yet similar tests (i.e., similar on content, and similar on when the tests were administered to similar sets of students) do not provide similar answers about teachers’ value-added performance. This is an issue with validity, in that, if a test is measuring the same things for the same folks at the same times, similar-to-the-same results should be realized. But they are not. Papay, rather, found moderate-sized rank correlations, ranging from r=0.15 to r=0.58, among the value-added estimates derived from the different tests.
Recently released, yet another study (albeit not yet peer-reviewed) has found similar results…potentially solidifying this finding further into our understandings about VAMs and their issues, particularly in terms of validity (or truth in VAM-based results). This study on “Comparing Estimates of Teacher Value-Added Based on Criterion- and Norm-Referenced Tests” released by the U.S. Department of Education and conducted by four