To adjust for clustering within
schools, injury rates and relative risks (RRs), along with their confidence intervals (CIs), were estimated using random-effect metaanalytic
methods as proposed by Thompson and colleagues.13
Expected post-intervention injury rates in the intervention
schools were calculated by multiplying the pre-intervention rates
in intervention schools by the ratio of post-intervention to preintervention
rates in the nonintervention schools. The proportion
of injuries prevented (etiological fractions)14 were calculated by
subtracting observed from expected rates and expressing the result
as a percentage of the expected rate. The analysis was repeated for
the subset of injuries flagged as equipment related.