This study tests whether changes in licensed nurse staffing led to changes in patient safety, using the natural experiment of 2004 California implementation of minimum staffing ratios. We calculated counts of six patient safety outcomes from California Patient Discharge Data from 2000 through 2006, using the Agency for Healthcare Research and Quality Patient Safety Indicators (PSI) software. For patients experiencing nonmortality-related PSIs, we measured mean lengths of stay. We estimated difference-in-difference equations of changes in PSIs using Poisson models and calculated the marginal impact of nurse staffing on outcomes from fixed-effect Poisson regressions. Licensed nurse staffing increased in the postregulation period, except for hospitals in the highest quartile of preregulation staffing. Growth in registered nurse staffing was associated with improvement for only one PSI and reduced length of stay for one PSI. Higher registered nurse staffing per patient day had a limited impact on adverse events in California hospitals.
Keywords: hospitals; nurse staffing; nursing; patient outcomes; quality; regulation.