For 90% power, 1049 children were
required in each arm. Using the practice as the unit of
analysis, we determined that 18 practices allowed
detection of a reduction in frequency of injuries of 16%
based on a mean injury rate of 0.32 (SD 0.05) and 80%
power. Eighteen practices (combined population of
1124 children in the intervention group and 1028
children in the control group) were therefore studied.