Quantitative systematic review of the transformational leadership style as a driver of nurses' organisational commitment

To explore the association between nurses' perceptions of their nurse manager's transformational leadership style and nurses' organisational commitment.


| INTRODUC TI ON
Nurse shortages are a global problem. In the US, it was predicted that there would be 0.8 million vacant nurse positions by 2020 (Hudspeth, 2013), and 0.59 million were predicted in Europe (Matrix Insight, 2012). Over the next decade, a shortage of 60,000 nurses is expected in Canada, a shortage of 90,000-109,500 is expected in Australia (Buchan et al., 2013), and a shortage of 1.59 million is expected in China (Liu & Yunibhand, 2016). The rapid growth of the older population is also a global problem. The Royal College of Nursing (Great Britain) (2017) has highlighted that ageing populations will worsen the existing nurse shortages. Population ageing can increase the demands on healthcare services, shrink the healthcare workforce (as existing nurses get older) (Beard & Bloom, 2015), and make it more difficult to recruit sufficient young nurses in the future. Moreover, the healthcare demands of the older population will both increase and change, with studies pointing out that many age-related diseases will increase, particularly circulatory, eye, and blood and blood-forming organ diseases (Vrhovec & Tajnikar, 2016;Yeo et al., 2016). This highlights that population ageing will increase the healthcare burden and worsen the existing nurse shortage.
To address the nurse shortage issue, a useful strategy is to increase the supply by increasing nurse training places and recruiting more nursing students; the other useful strategy is to ensure that the nurse retention rate is high (World Health organisation (WHO), 2006;Scott, 2017;European Commission, 2015). In recent years, many countries have increased the recruitment and training of new nurses. These strategies have had some positive effects.
However, they may not be sufficient to address the predicted lack of nurses in the coming years because of the lack of adequate numbers of faculty to train new students (Duvall & Andrews, 2010) and because nursing is often not very appealing to young people due to it being underpaid (Coombs et al., 2003). Therefore, more focus should be paid to retaining nurses if healthcare organisations hope to address the nurse shortages.
However, the percentages of existing nurses intending to leave their jobs is high (Aiken et al., 2013;National Council of State Boards of Nursing, 2010), and nurses' intention to leave their jobs is directly related to the turnover rate (Takase et al., 2015). Globally, many healthcare organisations' annual nurse turnover rates are as high as 20% (El-Jardali et al., 2009;Hayhurst et al., 2005). Copanitsanou et al. (2017) highlight that high turnover is an important reason underlying the nurse shortages. Moreover, studies have concluded that.
organisational commitment (OC) is a major factor that can reduce nurse turnover (Meyer & Herscovitch, 2001;Somers, 2009;Takase et al., 2015). Therefore, improving nurses' OC to reduce the turnover rate should be a primary focus. Meyer et al. (2002) found that effective leadership has a positive influence on OC. Effective leadership skills among nurse managers increase nurses' OC (and leadership skills are also useful more generally in healthcare) (Cummings et al., 2010). Effective leadership can improve the work environment by providing effective organisational support so that nurses' feel more valued in the team and their inner working lives are improved (Al-Hamdan et al., 2017;Sherman & Pross, 2010). A good work environment can improve job satisfaction and OC, thus helping to retain nurses (Aiken et al., 2013;Copanitsanou et al., 2017;Laschinger & Finegan, 2005;Li et al., 2018). Nurse managers have the responsibility for promoting care, ensuring patient safety, enhancing the quality of nurses' working lives, and managing the change processes related to all nursing care duties (Everson-Bates, 1992;McNeese-Smith, 1997;Sullivan & Garland, 2013). Therefore, it is important for nurse managers to adopt an effective leadership style to strengthen OC in order to reduce nurse turnover and to ensure safe and high-quality care.

| BACKG ROU N D
The transformational leadership (TFL) style is the preferred leadership style among nurse managers (Ferguson, 2015).
Transformational leadership has been the most influential leadership theory in recent decades (Judge & Piccolo, 2004). Transformational leadership (Bass, 1985) is a transformation process in which leaders inspire their followers to have the belief that they can reach exceptional achievements. Transformational leaders' long-term aims are to change and transform employees in their ways of thinking to achieve leadership effectiveness (Bass, 1985;Yukl, 1999). Therefore, the leader's transforming abilities are a key to the work of transformational leadership.
Transformational leadership is comprised of four components (Bass, 1985): (a) Idealised influence; (b) Inspirational motivation; (c) Individualised consideration; (d) Intellectual Stimulation. Idealised influence represents seeing the leader as a role model, thereby motivating the staff. Inspirational motivation represents leaders using the goals and sharing a vision to motivate the staff. Individualised consideration refers to leaders providing support to staff according to individuals' development needs, providing them with the greater opportunities to make decision, deal with challenges themselves, thereby helping them to achieve self-actualization. When it comes to intellectual stimulation, this refers to leaders encouraging staff to solve difficult problems themselves and think critically, thereby helping them more creatively. (Lussier & Achua, 2007).
Increasing nurses' OC is a major strategy to reduce the likelihood of nurses leaving their jobs (Meyer & Herscovitch, 2001;Somers, 2009;Takase et al., 2015). Therefore, it is important to identify the association between this style and nurses' OC. There K E Y W O R D S literature review, nurses, organisational commitment, systematic review, transformational leadership are many studies exploring the relationships between leadership styles and employees' OC. Most studies found that there is a positive relationship between them (Dai et al., 2013;Luo et al., 2017;Nguni & Denessen, 2006;Rowden, 2000;Yahaya & Ebrahim, 2016;Yousef, 2000). However, most of the research has been conducted in business and manufacturing settings, with fewer studies focusing on nurses. Therefore, the previous findings are not sufficient to show that there is a positive association between the two variables in the nursing care setting. Moreover, a study suggested that nurse executives' TFL style negatively affected OC (Leach, 2005). Therefore, the relationship between the TFL style and nurses' OC is unclear.
Notably, there has been no systematic review on the relationship between nurse managers' TFL style and nurses' OC. Therefore, we synthesised and analysed related studies (the literature was searched from January 2009 to December 2020) to identify the relationship between them.

| Aims
The aim of this systematic review was to synthesise and analyse information from the available quantitative studies on the association between nurses' perceptions of their nurse managers' TFL style and nurses' OC. The review question was: What relationship between nurse managers' TFL style and nurses' OC?

| Design
This study involved a quantitative systematic literature review, and the synthesis of the findings is presented in a narrative way.

| Search methods
To search for relevant studies achieving comprehensiveness of coverage and maintaining the precision of the records retrieved, the search strategy based on the MeSH keywords (nurses, transformational leadership, organisational commitment) to search the text word (free text), including all spelling variants (e.g. UK and US spelling), synonyms, abbreviations, relevant truncation, and wildcard, to expand the range of search terms (McGowan et al., 2016).
This research also used Boolean operators 'OR', and 'AND' to join the search term based on the logical relationship.
The search strategy in Table 1 was used to identify all relevant observational surveys published between January 2009 and December 2020 (to access the latest evidence) on CINAHL Complete, MEDLINE, PubMed, Business Source Complete, and Cochrane Library. In addition, it applied subject terms and English Language limitations in the search strategy. Hooton et al. (2007) highlight that the published studies are likely to may tend to report effective findings compared with grey literature including the unpublished study in a systematic review can help to avoid the reporting bias. Therefore, to gain as much information as possible and to minimise the reporting bias, the researchers (XHH and HUB) also searched on some valid websites, such as OpenSIGLE (http://www.openg rey.eu/), to find the grey literature. To obtain as many studies as possible, the reviewers manually searched the reference lists of the eligible studies. In addition, to check that there were no similar reviews already underway, the reviewers also searched the PROSPERO systematic review database.

| Participants
As this review aimed to explore the relationship between TFL style and nurses' OC, the participants of interest were registered nurses.

| Exposure factor
The nurses were exposed to nursing managers' TFL behaviour situ- E (exposure factor) Transformational leadership "Transformational leadership" OR "TFL" OR "Transformational leader*"OR "Transformational manager*" motivation, individualised consideration, and intellectual stimulation (Bass, 1985). Therefore, the nurses in this review were exposed to the nurse managers' idealised influence, inspirational motivation, individualised consideration, and/or intellectual stimulation.

| Outcomes
The primary outcomes of this review were nurses' OC, affective commitment (AC), continuance commitment (CC), or normative commitment (NC). As AC, CC, or NC to a particular organisation are encompassed by OC (Allen & Meyer, 1996;Mowday & Porter, 1979;Mowday & Steers, 1982), any study with results on OC, AC, CC, or NC were included in this review.

| Types of studies
Only observational studies were included, as experimental trials were excluded owing to ethical issues. Sullivan and Garland (2013) suggest that an effective healthcare manager should adopt a suitable leadership style based on the specific situation. Clinical contexts are complex, so it is impossible to use only a single consistent type of leadership (Sullivan & Garland, 2013). Hill (2017) also highlights that combining using the situational leadership style allows nurse managers to adopt an appropriate style for different staff and dynamic environments, thereby motivating staff to work more effectively.

| Language
In this systematic review, language was restricted to English.

| Search outcome
The literature search was independently conducted by two review-

| Study selection
According to the Cochrane Handbook (Higgins & Green, 2011), this process is also best done by more than one author, which can help to reduce the risks of some eligible reports being discarded. Two

| Quality appraisal
The Joanna Briggs Institute (JBI) Critical Appraisal Tool for Cross-Sectional Studies is a useful quality assessment tool that is wellaccepted by users (Munn et al., 2014). The Critical Appraisal Checklist for Cross-Sectional was applied to assess the bias, validity, limitations, and generality of cross-sectional studies (Munn et al., 2014).
Therefore, this tool was used to assess the included studies' quality. Two independent reviewers (Xiong and Huang) conducted the critical appraisal and reached a consensus. When no consensus was reached, a third author would be consulted.

| Data extraction
To reduce the error rate (Higgins & Green, 2011), two reviewers (XHH and HH) transparently and independently extracted the following data standardised data extraction tables developed in a pilot study: first author, year of publication, aims, study design; sample characteristics (sample selection; sample No.), study setting/context, measurement tools, reliability of the measurement tools, statistical analysis methods, and outcomes. When no consensus was reached, a third author would be consulted.

| Synthesis
The included studies exhibited considerable clinical, methodological, and statistical heterogeneity. aged care agency nurses, respectively, which caused clinical heterogeneity. Additionally, there were various sampling methods and study designs, and there were seven tools to assess TFL style, six tools to assess OC, and seven combinations of statistical methods to analyse the data, which caused large methodological and statistical heterogeneity. Higgins and Green (2011) suggest that the data synthesis in a systematic review should involve a narrative approach when the data cannot be combined or when the data exhibits large heterogeneity, because of the high risk of reaching a misleading conclusion when conducting a meta-analysis in cases involving large heterogeneity. Therefore, the data synthesis in this review involved a narrative approach. This review involved 2885 nurses, almost half of these participants (n = 1037) belonged to the study by Brewer et al. (2016).

| Characteristics of included studies
The study with the smallest sample size (n = 60) was by Al- Hussami (

| Tools to evaluate TFL style
Seven tools were used to evaluate the exposure factor, that is, nurses' perceptions of their nurse managers' TFL style ( Table 2). (2009)

| Tools to evaluate OC
Six tools were used to evaluate the outcome, that is, nurses' OC ( Table 2). Al-Hussami (2009)

| Summary of the statistical analysis methods
The statistical methods varied among the seven studies ( Table 2). (2009)

| Summary of study outcomes and data synthesis
All included studies demonstrate that there is a correlation  Table 2.
The result of the quality assessment from JBI Cross-sectional Critical Appraisal Tools of included articles is in Table 3

| DISCUSS ION
Nurse shortages are a global issue and ageing populations will worsen the existing issues. The high turnover rates among nurses contribute to nursing shortages. Training and recruiting more nursing students cannot fully address this problem. More effort should be made to retain nurses. A major factor underlying the retention rate is nurses' organisational commitment, which is related to effective leadership.
The transformational leadership style is the prevailing contemporary leadership style among nurse managers. However, it is controversial whether there is a positive or negative association between transformational leadership style and nurses' organisational commitment.
Therefore, we conducted this review to find evidence to explore the relationship between transformational leadership style and nurses' organisational commitment, thereby providing some suggestions for nursing managers to adopt an appropriate leadership style and improve nurses' organisational commitment. Meanwhile, it can help the hospital reduce the cost of nurses turnover and recruit new nurses. Moreover, addressing the nursing shortage can help to deal with the ageing population's healthcare problem in the long term.
This review found seven studies met the included criteria. All included studies found that there was an association between the TFL style and nurses' OC. Six studies reported that the TFL style had a positive effect on nurses' OC (Al-Hussami, 2009;Al-Yami & Watson, 2018;Brewer et al., 2016;Kodama et al., 2016;Lin et al., 2015;Xie et al., 2020). Only one study found that the association of the TFL style with nurses' OC was negative using Pearson's correlation analysis, but was positive using Stepwise linear regression, although not significant (Asiri et al., 2016). in Saudi Arabia, the results may only be generalizable to this unit or acute care nurses in Saudi Arabia. However, this negative finding was contrary to the other positive finding (Kodama et al., 2016) in this review which also studied the acute nurses, but in Japan.
These different results may be influenced by different cultural backgrounds. Furthermore, the study (Asiri et al., 2016) reported that the mean score of the nurses' perceptions of their nurse managers' TFL level was 2.55 (SD = 0.75) (Asiri et al., 2016). However, Bass and Avoliov (1997) reported that an effective TFL style required all component scores to be >3.0. This indicates that the nurse managers' TFL style in the study by Asiri et al. (2016) was not effective. Al-Hamdan et al. (2017) and Aboyassin and Abood (2013) highlight that ineffective leadership styles negatively affect employees' OC.
Therefore, the finding by Asiri et al. (2016) that there was a negative association between the TFL style and nurses' OC may have occurred because the nurse managers' TFL style was ineffective.
Moreover, Asiri et al. (2016)'s study was considered a piece of moderate-quality evidence (Table 3). Therefore, further research is required to support this conclusion, due to the lack of primary studies or systematic reviews.
There were six positive findings on the association between the TFL style and nurses' OC in this review. Two positive findings were focused on aged care nurses (Al-Hussami, 2009;Xie et al., 2020).
Both of them were moderate-quality evidence ( Table 3) The other three positive findings were from nurses from different nursing professions (general, paediatric, dental, diabetic, Nursing home, etc). One high-quality piece of evidence (    The remaining study (Kodama et al., 2016) found that the TFL style had a positive association with acute nurses' OC in Japan. This moderate-quality evidence ( Table 3) involved 369 acute nurses.
However, there was one study in this review (Asiri et al., 2016) found that the TFL style had a negative association with acute nurses' OC in Saudi Arabia. As mentioned above, further research is needed to address this controversial problem, due to the lack of sufficient evidence. Second, the exposure factor and the outcome were assessed using self-administered questionnaires in all included studies. There is a risk that a participant may not report the actual behaviours witnessed or their individual experiences and feelings (Polgar & Thomas, 2013), for example, they may misunderstand a question or feel pressure from their nurse manager to answer in a certain way.

| Limitations
This increases the risk of bias in this review. Additionally, as all the measurement tools assessed the participants 'perceptions of the nurse managers' TFL level or their feelings about OC, the answers were all subjective rather than objective. In addition, Al-Yami and Watson (2018) used the MLQ (Bass, 1985) to assess the outcome, and the MLQ has questionable validity, which negatively affected the study validity.
Third, the sampling methods used in the included studies also contribute to the limitations of this review (Bettany-Saltikov, 2012).  (2018) had unclear inclusion and exclusion criteria. Furthermore, Al-Hussami (2009) did not discuss why five participants' data were missing. All these factors also contribute to bias in this review.
Fourth, the studies by Al-Hussami (2009) and Asiri et al. (2016) did not provide details of research ethics committee approvalor participant informed consent, which increases the risk of ethical issues in this review.

| Strengths of the review
The strengths of this review include involving a total of 2885 par- Xie et al., 2020). This contributes to the generalisability of this review to both western and eastern countries.

| CON CLUS ION
In summary, all the included studies found there is an association between the TFL style and nurses' OC. Six studies consistently demonstrated that the TFL style was positively associated with nurses' OC, which is consistent with other studies in business (Dai et al., 2013;Luo et al., 2017). Only one study reported that the relationship between these two variables was negative in Pearson's correlation analysis, but was not significant positive in Stepwise linear regression; moreover, this study is not a strong evidence, and the finding may be due to the nurse managers' ineffective TFL style in the acute care unit or the culture influence. A further research is needed to address the whether the negative findings may be due to the nurse managers' ineffective TFL style or he culture influence.

AUTH O R CO NTR I B UTI O N S
All authors have agreed on the final version and meet at least one of the following criteria [recommended by the ICMJE (http://www. icmje.org/recom menda tions/)]: • substantial contributions to conception and design, acquisition of data, or analysis and interpretation of data; • drafting the article or revising it critically for important intellectual content.

ACK N O WLE D G E M ENTS
None.

FU N D I N G S TATEM ENT
None.

CO N FLI C T O F I NTE R E S T S TATE M E NT
No conflict of interest has been declared by the authors.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data openly available in a public repository that issues datasets: Asiri