Hypothesis: Teachers, individually, don't have statistically measurable effects. Students in a given cohort are mostly identical, and are mostly exposed to iden

Hypothesis: Teachers, individually, don't have statistically measurable effects. Students in a given cohort are mostly identical, and are mostly exposed to identical environments, such that influences on them average to some mean. As a result, minor influences, as long as they act reasonably consistently, are elevated to statistical significance.