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How Does a Value-Added Model Compare to the Colorado Growth Model?

Abstract

"We compare teacher evaluation scores from a typical value-added model to results from the Colorado Growth Model (CGM), which eight states use or plan to use as a component of their teacher performance evaluations. The CGM assigns a growth percentile to each student by comparing each student's achievement to that of other students with similar past test scores. The median growth percentile of a teacher's students provides the measure of teacher effectiveness. The CGM does not account for other student background characteristics and excludes other features included in many value-added models used by states and school districts. This may lend the CGM more transparency for educators than a value-added model, but at a possible cost of fairness to teachers. Using data from the District of Columbia Public Schools, we examine changes in evaluation scores across the two methods for all teachers and for teacher subgroups. We find that use of the CGM in place of a value-added model depresses the evaluation scores for teachers with more English language learner students and increases the evaluation scores for teachers of low-achieving students. Our analyses suggest that the results may be explained by how the CGM adjusts for prior achievement and its exclusion of other measures of student disadvantage."

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