Background: Educational and developmental psychologists often examine how groups change over time. Two analytic procedures -- analysis of covariance (ANCOVA) and the gain score model -- each seem well suited for the simplest situation, with just two groups and two time points. They can produce different results, what is known as Lord's paradox.…

Unlike in elementary school, high-school teacher effects may be confounded with both selection to tracks and unobserved track-level treatments. I document sizable confounding track effects, and show that traditional tests for the existence of teacher effects are likely biased. After accounting for these biases, high-school algebra and English…