Playground injury rates (injuries per 1000 children per month)
were compared at the intervention schools and at the nonintervention
schools before equipment removal and after equipment
replacement. Person-time denominators used the average yearly
enrollment in each school multiplied by the number of months
that the school was in session. The same 10-month calendar periods
were selected before and after the intervention to avoid bias
related to seasonal variation in injury. The pre-intervention period
was from September 1999 to June 2000, and the postintervention
period was from January 2002 to December 2002
(excluding July and August).