The Accuracy of Aggregate Student Growth Percentiles as Indicators of Educator Performance

Mean or median student growth percentiles (MGPs) are a popular measure of educator performance, but they lack rigorous evaluation. This study investigates the error in MGP due to test score measurement error (ME). Using analytic derivations, we find that errors in the commonly used MGP are correlated with average prior latent achievement: Teachers with low prior achieving students have MGPs that underestimate true teacher performance and vice versa for teachers with high achieving students. We evaluate alternative MGP estimators, showing that aggregates of SGP that correct for ME only contain errors independent of prior achievement. The alternatives are thus more fair because they are not biased by prior mean achievement and have smaller overall variance and larger marginal reliability than the Standard MGP approach. In addition, the mean estimators always outperform their median counterparts.