How large are teacher effects

It is widelv accepted that teachers differ in their effectiveness, yet the empirical evidence regarding teacher effectiveness is weak. The existing evidence is mainly drawn from econometric studies that use covariates to attempt to controlfbr selec tion effects that might bias results. We use data from a four-year experiment in which teachers and students were randomly assigned to classes to estimate teacher effects on student achievement. Teacher effects are estimated as between-teacher (but within-school) variance components of achievement status and residualized achievement gains. Our esti-mates of teacher effects on achievement gains are similar in magnitude to those of'previous econo-metric studies, but we find larger effects on mathematics achievement than on reading achievement. The estimated relation of teacher experience with student achievement gains is substantial, but is sta-tistically significant only,for 2nd-grade reading and 3rd-grade mathematics achievement. We also,find much larger teacher effect variance in low socioeconomic status (SES) schools than in high SES schools