In case you missed it, eduwonkette offered up an excellent post last month about the accumulation of value-added teacher effects. She describes a new NBER Working Paper by Brian Jacob, Lars Lefgren and David Sims which suggests that these effects may not as large as prior research has suggested. Here's why:"Our estimates suggest that only about one-fifth of the test score gain from a high value-added teacher remains after a single year. Given our standard errors, we can rule out one-year persistence rates above one-third. After two years, about one-eighth of the original gain persists.Interesting. If true, this study doesn't say that teachers don't matter, but perhaps less than some value-added proponents have suggested because students may only retain a fraction of the knowledge gained from having more effective teachers in consecutive years.
Our results indicate that contemporary teacher value-added measures may overstate the ability of teachers, even exceptional ones, to influence the ultimate level of student knowledge since they conflate variation in short-term and long-term knowledge.
Our results suggest some caution should be taken in focusing on such measures of teacher effectiveness. If value-added test score gains do not persist over time, adding up consecutive gains does not correctly account for the benefits of higher value-added teachers."
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