  This study examines whether unobserved factors substantially bias education
evaluations that rely on the Conditional Independence Assumption. We add 14 new
within-study comparisons to the literature, all from primary schools in
England. Across these 14 studies, we generate 42 estimates of selection bias
using a simple matching approach. A meta-analysis of the estimates suggests
that the distribution of underlying bias is centered around zero. The mean
absolute value of estimated bias is 0.03{\sigma}, and none of the 42 estimates
are larger than 0.11{\sigma}. Results are similar for math, reading and writing
outcomes. Overall, we find no evidence of substantial selection bias due to
unobserved characteristics. These findings may not generalise easily to other
settings or to more radical educational interventions, but they do suggest that
non-experimental approaches could play a greater role than they currently do in
generating reliable causal evidence for school education.
