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Mohammed MA, Laney D. Overdispersion in health care performance data: Laney's approach. Qual Saf Health Care 2006;15:383–4.
The journal apologises for an error that has occurred within this letter, where the text following the second equation (σz = …) should read as follows:
where N is the sum of ni and is the mean of zi. The 3‐sigma control limits for Laney's p′‐chart are thus given by ±3σpiσz, thereby accounting for within and between hospital variations.