Kendrick D, Royal S. Cycle helmet ownership and use; a cluster randomised controlled trial in primary school children in deprived areas (Arch Dis Child 2004;89:330–5). There were two errors in the sample size calculation for this article. The correct figures should be 93% (not 90% in the calculation for helmet owning) and 48.5% (not 44.5% in the calculation for helmet wearing). Therefore, the sample size calculation should read:

The study had 80% power to detect a difference in the percentage of children owning a helmet from 81% to 93% between the 2 treatment groups.

and

It had 80% power to detect a difference in the percentage of children always wearing a helmet from 34% to 48.5%, at the 5% significance level.