How Does Value Added Compare to Student Growth Percentiles?

Abstract

We compare teacher evaluation scores from a typical value-added model to results from the Colorado Growth Model (CGM), which 16 states currently 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 (or average) 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. Using data from the District of Columbia Public Schools (DCPS), we examine changes in evaluation scores across the two methods for all teachers and for teacher subgroups. We find that use of growth percentiles in place of value added would have altered evaluation consequences for 14% of DCPS teachers. Most differences in evaluation scores based on the two methods are not related to the characteristics of students’ teachers.