title: Nurse staffing and patient outcomes: Strengths and limitations of the evidence to inform policy and practice. A review and discussion paper based on evidence reviewed for the National Institute for Health and Care Excellence Safe Staffing guideline development.
abstract: A large and increasing number of studies have reported a relationship between low nurse staffing levels and adverse outcomes, including higher mortality rates. Despite the evidence being extensive in size, and having been sometimes described as "compelling" and "overwhelming", there are limitations that existing studies have not yet been able to address. One result of these weaknesses can be observed in the guidelines on safe staffing in acute hospital wards issued by the influential body that sets standards for the National Health Service in England, the National Institute for Health and Care Excellence, which concluded there is insufficient good quality evidence available to fully inform practice. In this paper we explore this apparent contradiction. After summarising the evidence review that informed the National Institute for Health and Care Excellence guideline on safe staffing and related evidence, we move on to discussing the complex challenges that arise when attempting to apply this evidence to practice. Among these, we introduce the concept of endogeneity, a form of bias in the estimation of causal effects. Although current evidence is broadly consistent with a cause and effect relationship, endogeneity means that estimates of the size of effect, essential for building an economic case, may be biased and in some cases qualitatively wrong. We expand on three limitations that are likely to lead to endogeneity in many previous studies: omitted variables, which refers to the absence of control for variables such as medical staffing and patient case mix; simultaneity, which occurs when the outcome can influence the level of staffing just as staffing influences outcome; and common-method variance, which may be present when both outcomes and staffing levels variables are derived from the same survey. Thus while current evidence is important and has influenced policy because it illustrates the potential risks and benefits associated with changes in nurse staffing, it may not provide operational solutions. We conclude by posing a series of questions about design and methods for future researchers who intend to further explore this complex relationship between nurse staffing levels and outcomes. These questions are intended to reflect on the potential added value of new research given what is already known, and to encourage those conducting research to take opportunities to produce research that fills gaps in the existing knowledge for practice. By doing this we hope that future studies can better quantify both the benefits and costs of changes in nurse staffing levels and, therefore, serve as a more useful tool for those delivering services.
authors.affiliation: University of Southampton, National Institute for Health Research Collaboration for Applied Health Research and Care (Wessex), United Kingdom. Electronic address: peter.griffiths@soton.ac.uk.
authors.forename: Peter
authors.initials: P
authors.lastname: Griffiths
authors.affiliation: University of Southampton, National Institute for Health Research Collaboration for Applied Health Research and Care (Wessex), United Kingdom.
authors.forename: Jane
authors.initials: J
authors.lastname: Ball
authors.affiliation: University of Southampton, Centre for Innovation and Leadership in Health Sciences, United Kingdom.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Drennan
authors.affiliation: University of Southampton, National Institute for Health Research Collaboration for Applied Health Research and Care (Wessex), United Kingdom.
authors.forename: Chiara
authors.initials: C
authors.lastname: Dall'Ora
authors.affiliation: University of Southampton, Centre for Innovation and Leadership in Health Sciences, United Kingdom.
authors.forename: Jeremy
authors.initials: J
authors.lastname: Jones
authors.affiliation: University of Southampton, Centre for Innovation and Leadership in Health Sciences, United Kingdom; Dipartimento di Scienze Economiche, Politiche e delle Lingue Moderne - Libera Università Maria Ss Assunta, Roma, Italy.
authors.forename: Antonello
authors.initials: A
authors.lastname: Maruotti
authors.affiliation: University of Southampton, Centre for Innovation and Leadership in Health Sciences, United Kingdom.
authors.forename: Catherine
authors.initials: C
authors.lastname: Pope
authors.affiliation: University of Southampton, National Institute for Health Research Collaboration for Applied Health Research and Care (Wessex), United Kingdom.
authors.forename: Alejandra
authors.initials: A
authors.lastname: Recio Saucedo
authors.affiliation: Inselspital Bern University Hospital, Nursing Research Unit, Bern, Switzerland; Institute of Nursing Science, Faculty of Medicine, University of Basel, Basel, Switzerland.
authors.forename: Michael
authors.initials: M
authors.lastname: Simon
mesh_terms: D003362:Cost-Benefit Analysis; D004471:Economics, Nursing; D004739:England; D006287:Health Planning Guidelines; D007297:Inpatients; D009729:Nursing / Q000592:standards; D009741:Nursing Staff, Hospital / Q000639:trends*; D010561:Personnel Staffing and Scheduling
keywords: Cost–benefit analysis; Diagnosis-related groups; Economics, nursing; Manpower; Medical staff; National health programs; Nursing personnel; Patient safety; Review, systematic