California's nurse-to-patient ratio law and occupational injury.

Objective: To determine whether state-mandated minimum nurse-to-patient staffing ratios in California hospitals had an effect on reported occupational injury and illness rates.
Methods: The difference-in-differences method was applied: The change in injury rates among hospital nurses after implementation of the law in California was compared to the change in 49 other states and the District of Columbia combined. Data were drawn from the US Bureau of Labor Statistics and the California Employment Development Department, including numerator estimates of injury and illness cases and denominator estimates of the number of registered nurses (RNs) and licensed practical nurses (LPNs) employed in hospitals. Confidence intervals (CIs) for rates were constructed based on assumptions that favored the null hypothesis.
Results: The most probable difference-in-differences estimate indicated that the California law was associated with 55.57 fewer occupational injuries and illnesses per 10,000 RNs per year, a value 31.6 % lower than the expected rate without the law. The most probable reduction for LPNs was 38.2 %. Analyses of CIs suggested that these reductions were unlikely to be due to chance [corrected].
Conclusions: Despite significant data restrictions and corresponding methodological limitations, the evidence suggests that the law was effective in reducing occupational injury and illness rates for both RNs and LPNs. Whether these 31.6 and 38.2 % reductions are maintained over time remains to be seen [corrected].