The issue of the adequacy of nurse staffing in hospitals and its impact on patient outcomes remains contentious. While there have been a large number of studies demonstrating an association of staffing levels and skills mix on a wide range of outcomes, including mortality, hospital-acquired infections and overall length of stay in patients in hospitals, the vast majority of these studies have been conducted comparing high-staffed hospitals to low-staffed hospitals.1–6 Concerns have been raised that other factors than staffing also differ between high-staffed and low-staffed hospitals that might contribute to the observed differences, and that staffing plays a smaller role than is suggested by these studies.

Settling the issue through a study by randomly assigning different staffing levels to hospitals or units seems very unlikely to occur on logistic grounds. And given the existing body of work research ethics committees would probably not approve such a study. What has proven feasible is utilising day-to-day variations in staffing and census across units within hospitals to assess the impact of low staffing on patient outcomes. Prior to the study by Griffiths et al in this issue of BMJ Quality & Safety,7 two published studies have used this methodology. Needleman et al 8 identified shifts by units with substantial shortfalls in professional nurse staffing from targets established by a large academic medical centre’s staff projection system, and examined the association of cumulative exposure to low-staffed shifts on patient mortality over a 5-year period for 40 units. They found a substantial increase in mortality associated each low-staffed shift to which a patient was exposed. They also found that the hazard of mortality was increased for shifts with substantially higher than average patient turnover, as turnover was not incorporated into the staffing system. Fagerström et al 9 used data from 36 units …