Abstract
ABSTRACTThis study shows value-added models (VAM) and student growth percentile (SGP) models fundamentally disagree regarding estimated teacher effectiveness when the classroom distribution of test scores conditional on prior achievement is skewed (i.e., when a teacher serves a disproportionate number of high- or low-growth students). While conceptually similar, the two models differ in estimation method which can lead to sizable differences in estimated teacher effects. Moreover, the magnitude of conditional skewness needed to drive VAM and SGP models apart often by three and up to 6 deciles is within the ranges observed in actual data. The same teacher may appear weak using one model and strong with the other. Using a simulation, I evaluate the relationship under controllable conditions. I then verify that the results persist in observed student–teacher data from North Carolina.
Highlights
This study shows value-added models (VAM) and student growth percentile (SGP) models fundamentally disagree regarding estimated teacher effectiveness when the classroom distribution of test scores conditional on prior achievement is skewed
This study identifies how two leading models used for teacher effectiveness measurement, value-added models (VAM) and student growth percentile (SGP) models, compare to each other and shows why they differ
While a number of studies have documented that differences exist between different VAM and SGP models, and Castellano and Ho (2015) showed these differences are largely caused by differences in the aggregation function used to attribute student performance to a particular teacher, this is the first study to identify and explore conditional skewness as the underlying factor that drives model differences and the first to quantify the practical magnitudes of these differences
Summary
This study identifies how two leading models used for teacher effectiveness measurement, value-added models (VAM) and student growth percentile (SGP) models, compare to each other and shows why they differ. Variations of both models are currently in use across the United States and a growing body of work has provided direct comparisons of their reliability and validity. Two of the most notable implementations are the Tennessee Value Added Assessment System (TVAAS) in Tennessee and an SGP model in Colorado (sometimes called the Colorado Model) These models are increasingly being used in human resource decisions and almost always used in conjunction with other measures of the teacher’s performance (typically accounting for 15%–50% of the evaluation)
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