Citation Network Analysis of Nurse Staffing Research from the Past Two Decades: 2000–2022

Studies have indicated that higher numbers of nurses regarding staffing ensure patient safety and a better practice environment. Using citation analysis, this study visualizes the landscape of nurse staffing research over the last two decades to show the overall publication trends, major contributors, and main research topics. We extracted bibliometric information from PubMed from January 2000 to September 2022. After clustering the network, we analyzed each cluster’s characteristics by keyword. A total of 2167 papers were considered for analysis, and 14 clusters were created. The analysis showed that the number of papers published per year has been increasing. Researchers from the US, the UK, Canada, Australia, and Belgium have led this field. As the main clusters in nurse staffing research during the past two decades, the following five research settings were identified: nurse outcome and patient outcome research in acute care hospitals, nurse staffing mandate evaluation research, nursing home research, and school nurse research. The first three clusters accounted for more than 80% of the total number of published papers, and this ratio has not changed in the past 20 years. To further develop nurse staffing research globally, evidence from other geographic areas, such as African and Asian countries, and from long-term care or community settings is necessary.


Background
Worldwide, approximately 20 million nursing staff, accounting for more than 70% of healthcare professionals, provide patient care on the front lines 24 h a day, 365 days a year [1].The efficient and effective allocation of nursing staff is a major component of establishing a quality healthcare system.Since early 2000, in the context of developing indicators for quality assurance, the nurse-to-patient ratio and skill mix have been considered as some of the nursing sensitivity indicators [2].Several studies on appropriate nurse staffing have been conducted since the landmark studies of the early 2000s [3,4].Analyses of these findings from systematic and umbrella reviews have shown robust evidence for an association between higher nursing staffing and a shorter length of stay, increased patient satisfaction, improved quality of nursing care, fewer readmissions, and reduced in-hospital mortality [5][6][7][8][9][10][11].
Concurrently, to ensure safe nurse staffing, several countries and states have introduced legislation or financial incentives related to nurse staffing ratios in hospitals and other facilities.California was the first US state to enact a minimum nurse-to-patient staffing ratio law in 2004 [12,13].The state currently requires a minimum ratio of one nurse to five patients in medical-surgical units and one nurse to two patients in critical care units.Massachusetts, USA, requires a 1:1 nurse-to-patient ratio in ICUs.The New York State enacted the "Safe Staffing for Quality Care Act" for acute care facilities and nursing homes in 2021.A roughly 2:1 patient-to-nurse ratio in intensive care units or nursing homes requires facilities to meet a minimum daily average of three and a half hours of nursing care per resident [14].In Australia, Victoria [15] and Queensland [16] have established regulations and laws regarding the minimum required nurse-to-patient ratio in hospitals, with one nurse to four patients for the day shift and one nurse to seven patients for the night shift.Japan [17] and South Korea [18] have also introduced nurse staffing levels into their financial schemes as patient-to-nurse ratios differentiate the reimbursement of nursing fees.In Japan, there are four categories of nurse staffing in hospitals as follows: the 7:1 patient-to-nurse ratio, 10:1, 13:1, and 15:1.In South Korea, nursing fees are determined by the hospital type and combination of patient-to-registered nurse ratios and patient-to-nursing assistant or ward staff ratios [17].The UK does not have legal nurse staffing mandates [19]; however, the National Institute for Health and Care Excellence has introduced guidelines for safe nurse staffing levels [20].
To improve nurse staffing policies, it is important to conduct policy evaluations of these regulations in various countries and/or states.In countries without legislation, it is necessary to undertake research on safe nursing placement that captures the characteristics of the country's healthcare system.However, a geographic bias has been indicated in previous research, which has mostly presented evidence from high-income countries [21,22].In addition, nurses are active in both hospitals and other areas [1], and there is a lack of evidence and regulations on safe nurse staffing in long-term care settings [23,24] and community settings [25,26].Appropriate nursing assignments are important not only in acute care settings in acute care hospitals but also in long-term care settings and community settings.
Recently, with the advancement of language processing technology, "bibliometric (or citation network) analysis", which analyzes bibliographic information, has become increasingly useful in showing the landscape and identifying the major topics of a health service research [27,28].In nursing research, the themes of "magnet hospital" [29], "nursing student education" [30], or "nurse resilience" [31] have been used to visualize research trends and issues.Regarding nurse staffing research, studies have synthesized the evidence using systematic and umbrella reviews that focus on specific settings and outcomes, for example, intensive care units [9] and mortality in acute care hospitals [6,8,32], and study designs, such as longitudinal studies [10].As the research on nurse staffing has grown enormously, it has become difficult for traditional manual literature review methods to provide a bird's-eye view of the entire research area and its trends and hot spots.Bibliometric (or citation network) analysis methods can contribute to uncovering research gaps within nurse staffing research that have not yet been identified and research that remains untapped.Identifying this research gap can ultimately contribute to developing healthcare systems that benefit patients and nurses worldwide.
Consequently, this study aimed to visualize the landscape of nurse staffing research over the past two decades.It used citation analysis to show the overall trends in the number of publications and major contributors and reveal the main research topics and the trends in these topics based on similarities in citation relationships.

Methods Section 2.1. Design and Data Collection
A citation network analysis was conducted.We extracted bibliometric information from PubMed.The search strategy was "nurse staffing" [Title/Abstract] OR "nurse workload" [Title/Abstract] OR "nurse workforce" [Title/Abstract] OR "nurse to patient" [Title/Abstract] OR "patient to nurse" [Title/Abstract] OR "nurse to bed" [Title/Abstract].We set the publication limit from 1 January 2000 to 30 September 2022 (Appendix A Table A1).
In the search process, unlike traditional literature review methods, all the literature that met the above search strategy was included in the citation analysis.No limitation was imposed on the study setting.Nursing positions or titles were included, ranging from general nursing staff to advanced practice nurses.We also included a broad range of the literature with titles or abstracts in English, even if the main text was in a different language.

Analysis 2.2.1. Citation Network Analysis
We used the "academic-landscape system" at the University of Tokyo, Japan (https: //academic-landscape.com/page/about?next=/ accessed on 1 October 2022) (Copyright © 2010-2013 Innovation Policy Research Center, The University of Tokyo.) for the analysis.This system was developed to visualize citation network analyses through the following procedures: (1) retrieving data, (2) determining the maximum connected component and network clustering, and (3) visualization [33].We performed citation analysis by combining direct citation relationships (when a paper directly cites other papers and analyzing these citation relationships helps us understand the direct connections between papers), co-citation relationships (when multiple papers cite the same other paper and these papers are in a co-citation relationship), bibliographic coupling (when using the same references in their citations, the similarity between papers can be assessed through bibliographic coupling), weighting functions (when representing relationships between papers, assigning weights to each relationship is often used, which allows the importance of different relationships to be considered), keyword similarity (cosine similarity) (by treating keywords in papers as vector representations and calculating the cosine similarity between keyword vectors, the similarity in the content between papers can be evaluated), and modularity maximization (a method that uses the network theory to identify modules or communities within a network) [34,35] In the clustering process, papers that have no direct citation relationship with other papers are automatically excluded from the analysis.
The size of the clusters indicates the number of papers included, and the spatial distance between clusters indicates the similarity of the content.Clusters that are far apart in spatial distance indicate independent research topics with little citation relationship.More details of the analysis are described in Kajikawa et al. [33,36].The schematic analysis procedure is illustrated in Appendix A, Figure A1.
After clustering the network, we analyzed each cluster's characteristics by the keywords with higher scores of term frequency-inversed cluster frequency (TF-ICF), titles, and abstracts in papers that were frequently cited by other papers in the cluster in which they were published.TF-ICF is a vector representation of natural language and is an effective method for extracting important words that characterize a cluster.The following equations calculated TF-ICF: TF-ICF = TF × ICF where TF = the term (keyword) counts in the cluster/total word counts in the cluster, ICF = log[1 − (the total number of clusters/number of cluster including the term)] The TF-ICF scores were higher for words that occurred more frequently only in the cluster.A higher TF-ICF score indicates more importance regarding the word that characterizes the cluster.

Analysis of Publication Trends, Authors, and Journals
We also carried out a trend analysis of the number of publications, authors, and journals for the articles included in our analysis.For the trend analysis of the number of publications, we visualised each cluster's annual trends and showed each cluster's rise and fall over the past 20 years.The affiliations and countries of the top 20 authors, in terms of the number of publications, were listed to reveal the geographical distribution of the nurse staffing studies.The top 20 journals in terms of the number of publications were listed to identify which areas of medicine, nursing, and public health more likely covered the nurse staffing issue.

Global Publication Trends from 2000 to 2022
Of the 2308 articles returned by the search strategy, 14 clusters were formed from 2167 papers based on citation relationships.In total, 141 articles were automatically dropped during the clustering process because they had no citation relationship with other articles.The overall number of papers published per year has been on the rise, starting with 26 per year in 2000 and gradually increasing to more than 100 per year in 2011 and more than 160 per year after 2020 (Figure 1a).
We also carried out a trend analysis of the number of publications, authors, and journals for the articles included in our analysis.For the trend analysis of the number of publications, we visualised each cluster's annual trends and showed each cluster's rise and fall over the past 20 years.The affiliations and countries of the top 20 authors, in terms of the number of publications, were listed to reveal the geographical distribution of the nurse staffing studies.The top 20 journals in terms of the number of publications were listed to identify which areas of medicine, nursing, and public health more likely covered the nurse staffing issue.

Global Publication Trends from 2000 to 2022
Of the 2308 articles returned by the search strategy, 14 clusters were formed from 2167 papers based on citation relationships.In total, 141 articles were automatically dropped during the clustering process because they had no citation relationship with other articles.The overall number of papers published per year has been on the rise, starting with 26 per year in 2000 and gradually increasing to more than 100 per year in 2011 and more than 160 per year after 2020 (Figure 1a).

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr. Linda Aiken from the University of Pennsylvania, USA; Dr. Douglas M. Sloane from the University of Pennsylvania, USA; and Dr. Peter Griffiths from the University of Southampton, UK. from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

Analysis of Authors and Journals
Table 1 shows the top 20 most productive authors and their most recent affiliations and countries.Nurse staffing articles were mainly written by authors in the USA, UK, Canada, Australia, and Belgium.The top three researchers who published over 40 papers were Dr Linda Aiken from the University of Pennsylvania, USA; Dr Douglas M. Sloane from the University of Pennsylvania, USA; and Dr Peter Griffiths from the University of Southampton, UK.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.
Table 2 shows the top 20 journals in terms of their number of publications related to nurse staffing.In descending order, the most publications were in the Journal of Nursing Administration, Journal of Nursing Management, and International Journal of Nursing Studies, which published over 80 related articles each.Some journals in the medical field, but not specifically the nursing field, were also ranked in the top 20, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.

The Clustered Network Map of Co-Cited References
Out of the 14 clusters, the top five clusters comprised 97.5%.The 6th to 14th clusters, thus, only consisted of a total of 54 papers (2.5%).Therefore, in this paper, we discuss only the top five clusters, which included a level of papers that could be interpreted (Figure 2).Cluster #1 was titled "nurse outcome research in acute care hospitals", Cluster #2 was titled "patient outcome research in acute care hospitals", Cluster #3 was titled "nurse staffing mandate evaluation research", Cluster #4 was titled "nursing home research", and Cluster #5 was titled "school nurse research".Over the past 20 years, the proportions of each cluster have not changed (Figure 1b).
Cluster #1, "nurse outcome research in acute care hospitals", was the largest, with 771 articles."ICU" (0.00171), "workload" (0.00075), "environment" (0.00061), "quality" (0.00050), and "outcome" (0.00045) were identified as representative key terms with high TF-ICFs (Table 3 and Appendix A Table A2).In this cluster, the papers mainly focused on the association between nurse staffing and nurses' reporting on the quality of care and nurse outcomes (burnout, intent to stay in their role, satisfaction) in critical care or acute care hospitals (Table 3).The annual number of publications showed an increase overall, although there have been various increases and decreases.This cluster represents an annual share of 30-40% (Figure 1).The annual numbers were around Cluster #2 was titled "patient outcome research in acute care hospitals" and comprised 614 articles."Mortality" (0.00127), "patient outcome" (0.00104), "hospital" (0.00103), "fall" (0.00078), "surgical" (0.00066), and "readmission" (0.00062) were identified as key terms with high TF-ICFs (Table 3 and Appendix A Table A2).In this cluster, the papers mainly focused on the association between patient outcomes (mortality, length of hospital stay, fall, readmission) in acute care hospitals.Similar to Cluster #1, the annual number of publications in this cluster showed an increase overall and comprised an annual share of Cluster #1, "nurse outcome research in acute care hospitals", was the largest, with 771 articles."ICU" (0.00171), "workload" (0.00075), "environment" (0.00061), "quality" (0.00050), and "outcome" (0.00045) were identified as representative key terms with high TF-ICFs (Table 3 and Appendix A Table A2).In this cluster, the papers mainly focused on TF-ICF: term frequency-inverse cluster frequency.
In terms of spatial location, Cluster #1 and Cluster #2, as well as Cluster #1 and Cluster #3, were found to be in close spatial distance and highly similar.Conversely, Clusters #4 and #5 were far from the other clusters and, thus, represented independent research topics (Figure 2).

Discussion
To the best of our knowledge, this study is the first to capture the landscape of articles on nurse staffing from 2000 to 2022 using a citation network analysis.In terms of the overall publication trends, we found that nurse staffing research has expanded, led by US and UK researchers.We also found that nurse staffing research can be divided into five main clusters according to the co-citation relationship: #1 "nurse outcome research in acute care hospitals", #2 "patient outcomes research in acute care hospitals", #3 "nurse staffing mandate evaluation research", #4 "nursing home research", and #5 "school nurse research".

Overall Publication Trends and Leading Authors and Journals from 2000 to 2022
The overall trend of annual publications has been rising, indicating that the nurse staffing research field has been growing.However, it is suggested that more robust evidence on new adequate nurse staffing methods is needed, for instance, flexible nurse staffing methods beyond the use of ratios [49], shift work, working time [50,51], and nurse allocation methodology [52,53].Thus, the theme of nurse staffing should continue to be a focus in the future.
Researchers from the US, the UK, Canada, Australia, and Belgium have led this field.This finding is consistent with the findings of a previous study focusing on the overall trend of high-impact nursing research papers, which are defined as papers ranked in the top 10% of citation frequency [30].Among the top 20 contributors, the most prolific can be divided into the following three groups: the first includes Linda H. Aiken and Douglas M. Sloane at the University of Pennsylvania, US; the second includes Peter Griffiths and Jane Ball at the University of Southampton, UK; and the third includes Joan Spetz and Charlene Harrington at the University of California system, US.As for the first group, the University of Pennsylvania was also ranked number one in high-impact nursing research [54]; researchers at the University of Pennsylvania are also the major contributors to, and leaders in, the magnet hospital research [29] and the huge international nurse staffing research group "RN4CAST" was launched in 2007 [55,56].As for the second group, Griffiths et al. (2016b) from the University of Southampton contributed to the adequate nurse staffing project at the National Institute for Health and Care Excellence in England, leading to the creation of nurse staffing policies in England [20,57].The third group, Spetz et al. (2004Spetz et al. ( , 2009) ) from the University of California, led an evaluation of the California nurse staffing mandate and nurse workforce research.
As for journals in which nurse staffing research is published, nursing administration journals are ranked highly.The International Journal of Nursing Studies, which has a broad scope across the nursing field, and other high-impact journals within the broader medical and health service research field also ranked highly, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.Higher attention to nurse staffing in nursing administration, health policy, and health service research is indicated.However, the publication trends and leading authors and journals over the past two decades demonstrate that there is still limited knowledge and evidence on adequate nurse staffing from other countries, such as low-middle income counties and/or Asian regions with different nurse staffing regulation systems within their healthcare systems.

Cluster by Citation Network Analysis
Five main clusters in nurse staffing research over the past two decades were identified in this study: #1 "nurse outcome research in acute care hospitals", #2 "patient outcome research in acute care hospitals", #3 "nurse staffing mandate evaluation research", #4 "nursing home research", and #5 "school nurse research".The first three clusters accounted for more than 80% of the total, and this share has not changed in the past 20 years.
The largest clusters were #1, "nurse outcome research in acute care hospitals", and #2, "patient outcome research in acute care hospitals".Although there was no small number of articles that examined both patient and nurse outcomes, those in Cluster #1, "nurse outcome research in acute care hospital", were relatively more focused on the working environment and nurses' turnover or retention in relation to the shortage in the nursing workforce.
Clusters #1, "nurse outcome research in acute care hospitals", and #2, "patient outcome research in acute care hospitals", were found to be spatially close, indicating the similarity in their context, with the only difference being whether they both focused on nurse outcomes or patient outcomes in nurse staffing research in acute care hospitals.Both clusters examine acute care hospital settings and reflect the trend of quality assurance in healthcare systems, mainly in the US, since the Institute of Medicine's "To Err is Human" report.To assess the extent to which nursing personnel in acute care hospitals contribute to healthcare quality, patient safety, and a professional and safe work environment, the National Quality Forum developed 15 performance measures for nursing-sensitive care in 2004 [2].Nursing sensitivity indicators included patient-centered outcomes, nursing-centered outcomes, and system-centered outcomes in nurse staffing and the Practice Environment Scale-Nursing Work Index (PES-NWI) [58].Our findings indicate an uptrend in Cluster #1, "nurse outcome research in acute care hospitals", after 2005, which the development of this nursing sensitivity outcome framework may explain.In addition, the uptrend in Cluster #2, "patient outcome research in acute care hospitals", since 2011, may have been affected by the progress in the international research project RN4CAST [55,56].Against the backdrop of worldwide nurse workforce shortages, RN4CAST was launched in 2007, coordinated by Walter Sermeus at Katholike Leuven, Belgium, and Linda Aiken at the University of Pennsylvania, to innovate forecasting methods by addressing the volume and quality of nursing staff as well as the quality of patient care.The project targeted 12 European countries, including the USA, Botswana, China, South Africa, and Chile.The findings from this project have been disseminated since 2011; this trend is shown in Figure 2c.Although the RN4CAST has a remarkable output, there are limitations in its cross-sectional design and manner of presenting the mean data, which do not consider the important differences in outcomes, staff characteristics, and care models [59].
Cluster #3 was titled "nurse staffing mandate evaluation research".The most famous minimum nurse staffing ratio mandate is that of California, US.Since the minimum nurse staffing mandate was introduced in 2004 and became active in 2005, it has been empirically examined as a natural experiment [13,42].Research articles that examined the expansion of mandates to other states then followed [40].The spatial distance between Clusters #1, "nurse outcome research in acute care hospitals", and #3, "nurse staffing mandate evaluation research", was also close.This may reflect the context in which the nurse staffing mandate in California, US, developed as a California Nursing Association initiative to improve the working environment for nurses and patient safety.Further, there are also some research articles from Australia in which minimum nurse staffing ratio mandates have been introduced [16].This trend is consistent with a previous systematic review that examined the evaluation of the nurse staffing methodology [60].Currently, Germany [61], Japan [17], and Korea [18] have reformed nurse staffing mandates and regulations in the payment system, although evaluations of related policy changes are still minimal compared to those in the US and Australia.Although healthcare systems differ from country to country, it is important to share knowledge about the various safe nurse staffing regulations, including laws, payment systems, and guidelines for nursing staffing, and their effects worldwide to build a better nursing delivery system.
In Cluster #4, "nursing home research", most articles were related to policy changes in Medicare Medicaid certification made by the Centers for Medicare Medicaid Service (CMS) in the US [62].The Nursing Home Compare website was launched in 1998 for quality assurance in nursing homes and has since undergone several revisions.The Nursing Home Compare program presents the following five categories of information: inspection results, including deficiencies from Medicare; certification surveys and complaint investigations; facility characteristics; nursing home staffing levels; and quality measures, which provide information on the clinical and physical characteristics of a nursing home's residents.This information is retrieved from the Online Survey Certification and Reporting (OSCAR) data and the Long-Term Care Minimum Data Set (MDS) Repository.In 2008, CMS launched the Five-Star Quality Rating System to help consumers, their families, and caregivers compare nursing homes more easily and identify areas about which they wanted to ask questions [63].In addition, since 2002, nursing home pay-for-performance programs, based on the quality of the chronic care delivered and using financial incentives tied to Medicaid or Medicare payment, have been implemented in some states and CMSs [44].Over the past two decades, approximately 10 papers have been published in this cluster annually.Understanding nurse staffing and its association with resident and nurse outcomes in long-term care settings is necessary.In addition, the importance of research in long-term care settings is even greater for both Asian countries with already super-aged societies and Asian and African countries that could become aged societies in the relatively near future.
Cluster #5, "school nurse research", was a relatively newly developed area of research.The annual number of papers was over five for the first time in 2018, and the total amount of research in this cluster is somewhat limited compared to research in acute care hospital settings.School nurses take on the role of case managers, bringing providers, families, and schools together to support students' health; as a result, better attendance and academic success are gained [64,65].Some states in the US recommend one school nurse for every 750 students in the healthy student population, a ratio of 1:225 for student populations requiring daily professional nursing services, a ratio of 1:125 for student populations with complex healthcare needs, and a ratio of 1:1 for individual students requiring daily, continuous professional nursing services [66].However, the National Association of School Nurses stated that the workload of school nurses has been expanding in line with an increase in children with mental health issues and those requiring special medical treatment [66].More evidence is necessary to ensure adequate and safe nurse staffing in school settings.

Limitations
This study has some limitations.The first relates to the search database.The WOS database is widely used in bibliometric analysis; however, we used PubMed because we could only access the current trends via WOS from 2000 to 2011 due to funding limitations.However, using PubMed has some benefits.For example, PubMed has the advantages of being the best database in the medical field, having an optimal update frequency, and including early online articles [67].Future studies should extend the search to WOS and other databases.Second, we could not include articles in which the title, abstract, and text were in a language other than English.Thus, we might have overestimated the geographic bias of the evidence on nurse staffing.Nevertheless, this citation analysis provides a more comprehensive picture of nurse staffing research in that, even if the text was in another language, it was included as long as some parts of the article, such as the title and abstract, were in English.Thus, this study included studies from the literature that have been excluded from traditional literature reviews.Third, since the method used in this study analyzes direct citation networks, the results may be biased due to the issue that papers already cited in other works in the literature are more likely to be cited in new papers.In recent years, it has been noted that evaluating researchers using the citation matrix is invalid as it depends on citation and publication counts [63].Alternative indexes considering co-author contributions and publication age beyond the sole publication count have been developed [68].Fourth, although we revealed a landscape of over 2000 nurse staffing articles from the viewpoint of the citation network, we could not evaluate the research quality of each article in the same manner as a systematic review.However, it is crucial to capture the latest research trends in real time from the rapidly growing academic literature.We believe that sharing the results of our analysis with the nursing community, including researchers, could provide objective evidence to help determine this community's future direction.

Conclusions
Using citation analysis, nurse staffing research over the past two decades formed five major clusters, depending on the study setting and outcomes focused on.This landscape of over 2000 nurse staffing articles revealed that evidence regarding long-term care settings and/or those from other geographic areas is still small compared to those in acute care settings from the US or UK.To ensure the safety of patients and nurses in all practice settings and locations, diverse geographic and setting knowledge is essential.

Figure 1 .Figure 1 .
Figure 1.Trends in the number of articles published from 2000 to 2022 (n = 2167).Cluster #1 was titled "nurse outcome research in acute care hospitals", Cluster #2 was titled "patient outcome research in acute care hospitals", Cluster #3 was titled "nurse staffing mandate evaluation research", Cluster #4 was titled "nursing home research", Cluster #5 was titled "school nurse re-Figure 1. Trends in the number of articles published from 2000 to 2022 (n = 2167).Cluster #1 was titled "nurse outcome research in acute care hospitals", Cluster #2 was titled "patient outcome research in acute care hospitals", Cluster #3 was titled "nurse staffing mandate evaluation research", Cluster #4 was titled "nursing home research", Cluster #5 was titled "school nurse research", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.
search", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.

Healthcare 2023 ,
11, x FOR PEER REVIEW 5 of 21 search", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.

Healthcare 2023 ,
11, x FOR PEER REVIEW 5 of 21 search", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.
Carolina at Chapel Hill USA Healthcare 2023, 11, x FOR PEER REVIEW 5 of 21 search", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.
and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.

Healthcare 2023 ,
11, x FOR PEER REVIEW 5 of 21 search", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.

Healthcare 2023 ,
11, x FOR PEER REVIEW 5 of 21 search", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.
search", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.
search", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.
11, x FOR PEER REVIEW 5 of 21 search", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.

Healthcare 2023 ,
11, x FOR PEER REVIEW 5 of 21 search", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022.

Figure 2 .
Figure 2. Visualization of the citation network of the top 5 clusters.Cluster #1 was titled "nurse outcome research in acute care hospitals", Cluster #2 was titled "patient outcome research in acute care hospitals", Cluster #3 was titled "nurse staffing mandate evaluation research", Cluster #4 was titled "nursing home research", and Cluster #5 was titled "school nurse research".The size of the clusters indicates the number of papers included, and the spatial distance between clusters indicates the similarity of the content.Clusters that are far apart in spatial distance indicate independent research topics with a limited citation relationship.

Figure 2 .
Figure 2. Visualization of the citation network of the top 5 clusters.Cluster #1 was titled "nurse outcome research in acute care hospitals", Cluster #2 was titled "patient outcome research in acute care hospitals", Cluster #3 was titled "nurse staffing mandate evaluation research", Cluster #4 was titled "nursing home research", and Cluster #5 was titled "school nurse research".The size of the clusters indicates the number of papers included, and the spatial distance between clusters indicates the similarity of the content.Clusters that are far apart in spatial distance indicate independent research topics with a limited citation relationship.

Table 1 .
Affiliations and countries of the top 20 authors.
, including Medical Care, Health Services Research, Health Affairs, BMJ Open, and the Journal of the American Geriatrics Society.16

Table 1 .
Affiliations and countries of the top 20 authors.

Table 1 .
Affiliations and countries of the top 20 authors.", and Clusters #6-14 are clusters that are small in number and were not included in the top five main clusters.(a) Trends in the number of publications from 2000 to 2022; (b) Trends in the proportions of each cluster from 2000 to 2022. search

Table 1 .
Affiliations and countries of the top 20 authors.
Matthew D. McHugh 36 University of Pennsylvania USA Joanne Spetz 27 University of California, San Francisco USA Sean P. Clarke 27 McGill University, Quebec Canada Charlene Harrington 25 University of California, San Francisco USA Walter Sermeus 23 KU Leuven-University of Leuven Belgium Eileen T. Lake 22 University of Pennsylvania USA Christine Duffield 21 University of Technology Sydney/Edith Cowan University Australia Jeannie P. Cimiotti 20 Emory University, Georgia USA athleen Rice Simpson 19 Mercy Hospital, Missouri USA Barbara A. Mark 19 University of North Carolina at Chapel Hill USA Vincent S. Staggs 15 University of Missouri-Kansas City, Missouri USA

Table 1 .
Affiliations and countries of the top 20 authors.
Christine Duffield 21 University of Technology Sydney/Edith Cowan University Australia Jeannie P. Cimiotti 20 Emory University, Georgia USA Kathleen Rice Simpson 19 Mercy Hospital, Missouri USA Barbara A. Mark 19 University of North Carolina at Chapel Hill USA Jane Ball 18 University of Southampton UK Peter I. Buerhaus 18 Montana State University USA Jack Needleman 18 University of California, Los Angeles, San Francisco USA Koen Van den Heede 17 KU Leuven-University of Leuven Belgium David W. Harless 16 Virginia Commonwealth University USA Vincent S. Staggs 15 University of Missouri-Kansas City, Missouri USA Eileen T. Lake 22 University of Pennsylvania USA Healthcare 2023, 11, x FOR PEER REVIEW 5 of 21

Table 1 .
Affiliations and countries of the top 20 authors.
University of California, Los Angeles, San Francisco USA Koen Van den Heede 17 KU Leuven-University of Leuven Belgium David W. Harless 16 Virginia Commonwealth University USA Vincent S. Staggs 15 University of Missouri-Kansas City, Missouri USA Christine Duffield 21 University of Technology Sydney/Edith Cowan University Australia

Table 1 .
Affiliations and countries of the top 20 authors.

Table 1 .
Affiliations and countries of the top 20 authors.
University of California, Los Angeles, San Francisco USA Koen Van den Heede 17 KU Leuven-University of Leuven Belgium David W. Harless 16 Virginia Commonwealth University USA Vincent S. Staggs 15 University of Missouri-Kansas City, Missouri USA

Table 1 .
Affiliations and countries of the top 20 authors.
Barbara A. Mark 19 University of North Carolina at Chapel Hill USA Jane Ball 18 University of Southampton UK Peter I. Buerhaus 18 Montana State University USA Jack Needleman 18 University of California, Los Angeles, San Francisco USA Koen Van den Heede 17 KU Leuven-University of Leuven Belgium David W. Harless 16 Virginia Commonwealth University USA Vincent S. Staggs 15 University of Missouri-Kansas City, Missouri USA

Table 1 .
Affiliations and countries of the top 20 authors.
Barbara A. Mark 19 University of North Carolina at Chapel Hill USA Jane Ball 18 University of Southampton UK Peter I. Buerhaus 18 Montana State University USA Jack Needleman 18 University of California, Los Angeles, San Francisco USA Koen Van den Heede 17 KU Leuven-University of Leuven Belgium David W. Harless 16 Virginia Commonwealth University USA

Table 1 .
Affiliations and countries of the top 20 authors.
University of California, Los Angeles, San Francisco USA Koen Van den Heede 17 KU Leuven-University of Leuven Belgium David W. Harless 16 Virginia Commonwealth University USA Vincent S. Staggs 15 University of Missouri-Kansas City, Missouri USA

Table 1 .
Affiliations and countries of the top 20 authors.
University of Technology Sydney/Edith Cowan University Australia Jeannie P. Cimiotti 20 Emory University, Georgia USA athleen Rice Simpson 19 Mercy Hospital, Missouri USA Barbara A. Mark 19 University of North Carolina at Chapel Hill USA Jane Ball 18 University of Southampton UK Peter I. Buerhaus 18 Montana State University USA Jack Needleman 18 University of California, Los Angeles, San Francisco USA

Table 1 .
Affiliations and countries of the top 20 authors.
University of Technology Sydney/Edith Cowan University Australia Jeannie P. Cimiotti 20 Emory University, Georgia USA athleen Rice Simpson 19 Mercy Hospital, Missouri USA Barbara A. Mark 19 University of North Carolina at Chapel Hill USA Jane Ball 18 University of Southampton UK Peter I. Buerhaus 18 Montana State University USA

Table 1 .
Affiliations and countries of the top 20 authors.
University of Technology Sydney/Edith Cowan University Australia Jeannie P. Cimiotti 20 Emory University, Georgia USA athleen Rice Simpson 19 Mercy Hospital, Missouri USA Barbara A. Mark 19 University of North Carolina at Chapel Hill USA

Table 1 .
Affiliations and countries of the top 20 authors.
Eileen T. Lake 22 University of Pennsylvania USA Christine Duffield 21 University of Technology Sydney/Edith Cowan University Australia Jeannie P. Cimiotti 20 Emory University, Georgia USA

Table 2 .
Titles and Journal Impact Factor TM of top 20 journals.
Journal Impact Factor TM was taken from the Web of Science (Clarivate).

Table 3 .
Representative keywords and papers in the top five clusters.