Safe staffing: evaluating the evidence for mandatory nurse-to-patient ratios
Research developed with the University of Southampton
We recently commissioned the internationally leading Health Workforce and Systems Research Group at the University of Southampton to conduct independent analysis on mandatory minimums and safe staffing legislation.
This evidence-based brief pulls together the conclusions from their findings.
There is a substantial and growing body of evidence demonstrating that registered nurse (RN) staffing levels have a significant impact on both patient and nurse outcomes.
Inadequate staffing has been linked to increased risks of patient harm, including preventable complications and in-hospital mortality. Harms to staff include burnout, occupational injury and lower job satisfaction.
Hundreds of studies – many involving large datasets across multiple hospitals and millions of patients – have been conducted internationally, including in the UK. While most of this research is observational rather than experimental, and not all studies find statistically significant effects, the overall pattern is clear and compelling.
Systematic reviews and meta-analyses consistently conclude that lower nurse staffing levels are associated with worse outcomes. Although the absolute effect sizes are sometimes modest, the scale of impact across health systems is substantial.
The consistency of findings across diverse settings strengthens the case for a causal relationship. In short, while no single study is definitive, the cumulative evidence strongly supports the conclusion that adequate nurse staffing is essential for patient safety and quality of care.
Mandatory minimum staffing levels are one possible response, but opponents argue that such policies are inflexible, costly and do not represent a cost-effective means of ensuring patient safety. Others argue that an exclusive focus on staff numbers in isolation risks downgrading the skills of the nursing team with less experienced or less well-trained staff being used in preference because of lower costs.
What does the evidence say?
Since the US state of California passed a law enshrining mandatory minimum Registered Nurse Staffing Levels for hospitals into law in 1999, there has been ongoing interest in implementing similar regulations in other jurisdictions. Despite this, progress has been slow.
While advocates cite a large body of evidence demonstrating that low staffing is linked to a range of serious adverse outcomes for patients and staff, opponents argue that such policies are inflexible, costly and do not represent a cost-effective means of ensuring patient safety. Others argue that an exclusive focus on numbers risks downgrading the skills of the nursing team with less experienced or less well-trained staff being used in preference because of lower costs.
In this brief we give an overview of the evidence about mandatory minimums and safe staffing legislation.
Staffing levels
There is evidence that the implementation of mandated minimum nurse-to-patient ratios achieved the policy aim of reducing the number of patients per nurse (such as improving staffing). Post-implementation patient-to-nurse ratios were reduced on average by one across all California hospitals.
However, increases in RN hours per patient have, at least until recently, been observed in most countries, even those with no safe staffing regulations. Analysis of US data confirmed that RN staffing increases in California were larger than in other US states and that the largest increases were seen in the hospitals with lowest baseline staffing.
Staff mix
Evidence for the benefits of increased staffing is largely based on increases in registered nurses. Where other staff groups (such as nursing assistants or other grades of qualified nurses, such as the US Licensed Practical Nurse) are considered, evidence is much less clear cut.
Overall evidence suggests that if the proportion of registered nurses were reduced because of a mandatory minimum policy, patient outcomes could worsen.
There is some evidence that Californian hospitals that had the largest staffing shortfalls prior to legislation relied more heavily on licensed practical nurses and licensed vocational nurses to make up staffing shortfalls, with potential unintended consequences for quality of care.
Maintaining mandatory minimum staffing levels can be achieved through increased baseline staffing plans or by increased use of temporary staff to avoid shortfalls in the face of variable demand.
A recent observational study of 626,313 admissions to 185 wards in four NHS hospitals found that use of temporary staff, whether bank or agency staff, was less effective than using permanently employed staff. Increasing permanent staff to avoid low staffing reduced the hazard of death by 7.7%, whereas using temporary staff reduced it by 4.1%.
Cost-effectiveness
While increases in registered nurse staffing levels linked to staffing mandates are likely to lead to improvements in outcomes for patients and staff, staffing costs will also be increased.
However, a review of 23 economic studies related to nurse staffing levels concluded that increases in the numbers of registered nurses could be cost effective, with a low cost per quality adjusted life year (£10,000 or less), a level regarded as 'exceptional value for money' by NICE when assessing new drug treatments. In contrast most studies in the review indicated that reduced skill mix led to worse outcomes at increased net cost.
This potential is confirmed by economic modelling from a recent study in four NHS hospitals, which estimates that eliminating registered nurse staffing in these hospitals would cost only £2,701 for each quality adjusted life year gained for patients. The value of reduced hospital stays, staff sickness and re-admissions exceeded the costs of additional staff, meaning that there was a net cost saving.
Direct evidence about implementing mandatory minimums is scarce. In the Australian state of Queensland, it was estimated that meeting RN ratio requirements would save 145 lives, avoid AU$69m (about £32m) costs from reduced lengths of stay and readmissions for a cost of AU$33 (£16m) to employ additional staff.
California experienced additional staff costs due to wage inflation caused by market forces, as hospitals employed more nurses following legislation. However, there was no clear evidence of an adverse effect of operating margins attributable to the policy, with one study concluding that any effect was 'marginal'.
Nurse outcomes
Reviews of evidence indicate that nurses’ burnout and job dissatisfaction were reduced in California following the implementation of mandatory staffing minimums, and lower where nurses’ workloads were in line with California-mandated ratios in other states.
Lower rates of occupational injuries and illness were observed in California compared to 49 other states without mandatory ratios.
Alternatives
Use of evidenced-based tools, such as patient classification systems, for example the Safer Nursing Care Tool, has been advocated as a more cost-effective alternative to mandatory minimum staffing levels, because tools allow staff to be deployed based on assessed need.
Reviews of evidence for the use of such tools found no evidence for the effectiveness or cost-effectiveness of such tools except in so far as they serve to increase overall staffing numbers.
It is worth noting patient classification systems establish an effective minimum staffing requirement (based on the staffing requirement associated with the lowest classification) and that California’s staffing legislation also mandates the use of a patient classification system to determine additional staffing requirements over and above the specified minimum.
The use of patient classifications was enshrined in law before the minimum staffing legislation was passed in 1999. Thus, the two approaches are not as different as they may appear, but mandating a patient classification system was not seen as sufficient protection in California.
While patient classification systems offer the promise of improved targeting of resources, to individuals or wards that are most in need, our economic model indicated that avoiding understaffing for the general population was more cost-effective than targeting particular high-risk groups.
Similarly while patient classification systems may help guide deployment of scarce resources in the face of staff shortages, having an adequate baseline staffing level reduced risk to patients at a low cost compared to strategies that relied heavily in temporary deployment, where risk to patients was increased and opportunity to save on staff cost was limited.
Conclusions
- Minimum staffing ratios are likely to improve patient and staff outcomes if baseline staffing levels are increased.
- Minimum staffing levels need to be used in conjunction with an evidenced-based tool and are not an alternative.
- Recent economic analysis in the NHS suggests that investing in better ratios is likely to be highly cost effective and may lead to cost savings due to reduced length of stay and readmissions.
- Given the overwhelming evidence that insufficient registered nurse staffing leads to patient harm and nurses’ turnover and dissatisfaction, policy efforts to guard against understaffing and skill-mix dilution are warranted.
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- An act to add Section 2725.3 to the Business and Professions Code and to add Section 1276.4 to the Health and Safety Code, relating to health care., in AB 394. 1999, State of California State of California.
How to cite this research: Dall’ora, C and Griffiths, P (2025) Safe Staffing: Evaluating the evidence for mandatory nurse to patient ratios. London: RCN. Available at: rcn.org.uk/Professional-Development/publications/rcn-safe-staffing-uk-pub-012-306 (accessed 21 October 2025)
An independent analysis by the University of Southampton found strong and consistent evidence that higher registered nurse staffing levels significantly improve patient safety and nurse wellbeing, with inadequate staffing linked to increased risks of harm and burnout.
This report shows that while mandatory minimum staffing levels are one potential solution, critics argue they may be inflexible and costly, and could inadvertently reduce the overall skill mix of nursing teams.
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Page last updated - 30/10/2025
