Original Research

The relationship between perceived organisational support, organisational commitment and turnover intention among employees in a selected organisation in the aviation industry

Maahierah Satardien, Rukhsana Jano and Bright Mahembe

Received: 11 Sept. 2018; Accepted: 06 Mar. 2019; Published: 05 Sept. 2019

Copyright: © 2019. The Author(s). Licensee: AOSIS.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Orientation: Turnover intention, a precursor of employee turnover, comprises some of the challenges experienced by the 21st-century organisation – the aviation industry has not been spared.

Research purpose: The primary goal of this study was to conduct an analysis of the relationships between perceived organisational support (POS), organisational commitment and turnover intention at a selected organisation within the aviation industry.

Motivation for the study: Relatively, little attention has been directed at examining employees’ POS and organisational commitment on turnover intention in the aviation sector.

Research approach/design and method: A quantitative research design was used. The sample composed of employees (N = 240) employed at an international aviation organisation, which operates in Cape Town, Brno and Istanbul. The participants were drawn using convenience sampling. Data were analysed using the Pearson correlation and multiple regression techniques.

Main findings: High levels of reliability were found for the scales used in this study. Significant correlations were found among POS, organisational commitment and turnover intention. A linear regression analysis indicated that both organisational commitment and POS are significant predictors of turnover intention.

Practical/managerial implications: The findings of this study suggest that POS and commitment play a significant role in reducing turnover intention. Employee support programmes should be offered as a panacea for turnover intention given the need for employee retention and commitment in this industry.

Contribution/value-add: The aviation industry strives to retain employees, by investing in employees’ training and development. The contribution of the study can ensure passenger safety and avoid aircraft disasters. Perceived organisational support is likely to enhance commitment and reduce staff turnover.

Keywords: perceived organisational support; organisational commitment; turnover intentions; employee turnover; staff retention; intention to leave.

Introduction

Turnover intention is one of the widely studied areas in the field of industrial psychology (Alkahtani, 2015; Fortuin, 2017; Hendricks, 2017; Mthembu, 2017). This is attributed to its impact on the operational costs of employing and re-employing employees. These costs include, but are not limited to, recruitment costs, leave capitalisation, relocation costs as well as formal training and induction costs (Alkahtani, 2015). In addition, organisations are losing their most valued intellectual capital to their competitors, which greatly affect their comparative advantage as some of the organisations’ ways of doing business might be shared with their rivals. Turnover intention also impacts internal employees as well as employee–customer relationships (Aliyu & Nyadzayo, 2018). In view of the possible impact of staff turnover, organisations need to foster behaviours and systems that promote staff retention. Aside from the common financial interventions, perceived organisational support (POS) plays a significant role in influencing turnover intention.

Perceived organisational support is widely defined as the employees’ ‘beliefs concerning the extent to which the organisation values their contribution and cares about their well-being’ (Eisenberger, Huntington, Hutchison & Sowa, 1986, p. 501). To date, POS has been documented to enhance job satisfaction (Zumrah & Boyle, 2015), organisational citizenship behaviour (Isfahani & Rezaei, 2017), job performance (Miao & Kim, 2010), corporate entrepreneurship (Rutherford & Holt, 2007) and entry-level employee innovativeness (Mummedy, 2008). Some of the benefits of positive POS include the inculcation of elevated levels of employee trust and organisational identification (Rhoades & Eisenberger, 2002), which, in turn, reduce turnover intention (Paille, Bourdeau, & Galois, 2010). Hence, the more an organisation expends some reasonable efforts towards the professional development of its human and intellectual capital, the more the employees feel the obligation to stay and help the organisation achieve its investment goals (Paille et al., 2010). On the contrary, low levels of POS are likely to be met with reduced morale, employee withdrawal symptoms and ultimately strong intentions to leave the organisation (Wayne, Shore, & Linden, 1997). Rhoades and Eisenberger’s (2002) meta-analysis also showed a strong negative relationship between POS and turnover intention. Besides reduce turnover intention, POS creates feelings of commitment in employees. Most studies point out that POS is a powerful predictor of organisational commitment (Kim, Eisenberger, & Baik, 2016; Makanjee, Hartzer, & Uyz, 2006; Naotunna & Arachchige, 2016).

According to Rhoades and Eisenberger (2002), the turnover–POS relationship is anchored on the fact that employees’ high POS levels develop feelings of attachment to the organisation characterised by an enhanced sense of control, purpose and meaning as they perform their roles. When employees feel that the organisation appreciates them by caring about their well-being, they are likely to develop some strong feelings of commitment which reduce turnover intention (Foong-Ming, 2008; Mabasa, Ngirande, & Shambare, 2016; Tsui, Pearce, Porter, & Tripoli, 1997; Tumwesigye, 2010).

Although several studies on the influence of POS on commitment and turnover intention exist (Fortuin, 2017; Islam, Khan, Ahmed, Ali, Ahmed, & Bowra, 2013; Kalidass & Bahron, 2016), most of the studies were conducted using employees from the corporate, hotel and tertiary institutions. For example, Islam et al. (2013) investigated the nature of the relationships among these variables in the banking sector in Malaysia; Kalidass and Bahron (2016) used a sample from the hotel industry and Fortuin (2017) used employees working in academia. Relatively, little attention has been directed at examining employees’ POS and organisational commitment on turnover intention in the aviation sector.

Research purpose and objectives

Thus, based on the discussion presented, the primary goal of this study was to conduct an analysis of the relationships between POS, organisational commitment and turnover intention at a selected organisation within the aviation industry. The secondary goal was to determine if organisational commitment and/or POS predict turnover intention.

Literature

Conceptualising turnover intention

The terms ‘intention to leave’ and ‘staff retention’ have been used interchangeably in existing research studies to indicate employee turnover behaviour. Turnover intention has been defined as voluntary, conscious and relatively strong recurring plans on the part of the employee to deliberately leave an organisation (Mobley, Horner, & Hollingsworth, 1979). These recurring thoughts and plans to leave the organisation trigger a cognitive process that is reinforced by factors occurring within and outside an organisation that result in the final decision to leave the organisation. This withdrawal cognition process incorporates the following aspects: (1) thoughts of leaving the organisation, followed by (2) the active search for another job elsewhere and (3) the final intention to leave or quit (Mobley et al., 1979). Turnover intention is arguably the best predictor of staff turnover (Griffeth, Hom, & Gaertner, 2000). This intention is often difficult for organisations to uncover unless the employee himself or herself shares with members of the organisation.

Conceptualising perceived organisational support

The term POS was coined in 1986 by Eisenberger et al. (1986). Eisenberger et al. (1986) define POS as relating to employees’ perception that the organisation cares about their well-being and development and values their contributions. Employees have a tendency to perceive their managers’ actions as personification of the actions of the organisation (Eisenberger, Armeli, Rexwinkel, Lynch, & Rhoades, 2001). Hence, the behaviour of the managers has some implications on the job performance of the employees as employees tend to perform better to pay back for POS, indicating an exchange relationship (Allen, Shore, & Griffeth, 2003; Aselage & Eisenberger, 2003; Dawley, Houghton, & Bucklew, 2010; Rhoades & Eisenberger, 2002).

Perceived organisational support has its roots in the norm of reciprocity (Rhoades & Eisenberger, 2002), the Social Exchange Theory and the Organisational Support Theory. The norm of reciprocity is anchored on two demands, namely, people should help those who have helped them, and people should not injure those who have helped them (Gouldner, 1960). This norm emphasises the exchange nature of the relationship. The Social Exchange Theory implies that individuals who receive greater support from their organisations will be more inclined to return the act of goodwill (Eisenberger, Lynch, Aselage, & Rohdieck, 2004). The Organisational Support Theory maintains that employees strive to pay back the organisation for the support rendered to them by increasing their efforts to ensure that the organisation meets its goals (Scott, Restubog, & Zagenczyk, 2013).

Organisational commitment

Numerous studies have been published on the organisational commitment construct (Adzeh, 2013; Mahembe, 2010; Mahembe & Engelbrecht, 2013), with the three-component model of organisational commitment developed by Allen and Meyer (1990) being the widely used definition of organisational commitment. These authors defined organisational commitment as a latent variable made up of affective, continuance and normative commitment as its manifest variables. Affective commitment has been identified in several studies as the best predictor of organisational commitment, which emphasises that employees continue working for the organisation because they want to (Meyer & Allen, 1991). It personifies the emotional attachment to the organisation developed over the years. In contrast, continuance commitment is based on an individual assessment of the perceived cost of leaving the organisation (Meyer & Herscovitch, 2001). In other words, this cost–benefit analysis leads to employees remaining because they feel they need to or have to (Meyer & Allen, 1997). On the contrary, normative commitment alludes to employees’ feeling of obligation to continue providing their services to the organisation because they feel it is the ‘moral’ thing to do (Allen & Meyer, 1990; Martin & Roodt, 2008; Meyer & Allen, 1991, 1997).

The relationships between perceived organisational support, organisational commitment and turnover intention

Turnover intention and perceived organisational support

An important consequence of POS that has been assessed in the literature, and which is relevant to this study, is its relationship to turnover intention (Rhoades & Eisenberger, 2002; Wayne et al., 1997). Rhoades and Eisenberger (2002) found the relationship between POS and intention to leave to be the strongest among the withdrawal behaviour variables included in POS studies. Existing studies on the relationship between POS and intention to leave an organisation indicate an inverse association, signifying that employees who feel supported by their organisations rarely think of quitting their organisations (Allen et al., 2003; Paille et al., 2010). The employee feels that the organisation pays attention to their self-development through various organisational initiatives (such as on the job coaching) meant to benefit the employee (Paille et al., 2010).

Turnover intention and organisational commitment

According to Muchinsky (2005), organisational commitment is one of the widely studied constructs in psychology, with numerous research studies spanning over 30 years appearing in different journals and books. Most of the studies indicate a significant negative relationship between the two constructs (Clayton & Hutchinson, 2002; Lee, 2000; Mathieu & Zajac, 1990). The findings indicate that the higher the organisational commitment, the lower the turnover intention. Several other studies have also associated a high organisational commitment with a high probability that an employee will stay with the organisation, perform their job at an optimal level and avoid being absent at work (Elangovan, 2001; Gaan, 2007; Lee, 2000; Luna-Arocas & Camps, 2008).

Turnover intention, organisational commitment and perceived organisational support

Perryer, Jordan, Firns and Travaglione (2010) observed that commitment and POS were important predictors of turnover intention. Employees with high levels of organisational support, but low levels of commitment, will be less inclined to leave the organisation. Hom and Griffeth (1995) suggest that organisational commitment increases the relationship between perceived organisational commitment and turnover. Turnover intention has been found to be an outcome of organisational commitment (Blau, 2000). In addition, researchers have substantiated the relationship between affective commitment and POS in the turnover process (Wayne et al., 1997).

From the foregoing discussion, the following hypotheses were postulated to guide the study:

• There is a negative relationship between employees’ perceptions of organisational support and turnover intention.
• There is a negative relationship between employees’ organisational commitment and turnover intention.
• There is a positive relationship between employees’ organisational commitment and POS.
• Organisational commitment and POS predict turnover intention.

Method

Participants

The population comprises permanent and contract employees (N = 240) employed at the aviation organisation as load control agents (47.9%) and support staff (52.1%). This international aviation organisation consists of three centres, which operate in Cape Town, Brno and Istanbul. Employees at all the three centres were invited to participate in the study. One hundred and forty-four online responses were received. However, of these 144 online responses, 131 were valid, yielding a 55% response rate.

The sample consisted of 78 (54.2%) women and 66 (45.8%) men. The majority of the participants were in the age category of 20–30 years (45.15%). Most of the respondents have been with the organisation for 4+ years (42.3%). The location of respondents was Cape Town (56.3%), Brno (29.2%) and Istanbul (14.6%).

Procedure

Because the study sites were spread across three countries (Brno, Cape Town and Istanbul), an Internet-based survey was considered to be the most appropriate method to gather the data. The organisation’s internal communication process was followed to invite staff to participate in the research survey. An email was sent to each of the employees in all the three centres as they all had access to the work email and Internet. The email contained a hyperlink inviting participants to complete the online survey.

Instruments

A self-developed questionnaire was used to acquire demographic information from the sample. Participants were requested to provide information with regard to their gender, age, marital status, years of service in the organisation, job level and whether they were employed in Cape Town, Brno or Istanbul.

Perceived organisational support

A shortened version of Eisenberger et al.’s (1986) 36-item Survey of Perceived Organisational Support was used to measure employees’ POS. The abbreviated POS measure contains 17 items scored using a seven-point Likert scale, ranging from 1 (strongly disagree) to 7 (strongly agree). Example items include ‘The organisation values my contribution to its well-being’ and ‘The organisation strongly considers my goals and values’. Previous studies that have used the 17-item version of the Survey of Perceived Organisational Support reported Cronbach’s alpha values of between a = 0.74 and a = 0.95 (Fields, 2002).

Organisational commitment

The 24-item Organisational Commitment Survey, developed by Meyer and Allen (1997), was used to measure organisational commitment. The scale measures the three components of organisational commitment, namely, affective, continuance and normative commitment, using a five-point Likert scale ranging from 1 (strongly disagree) to 5 (strongly agree). Each of the subscales contains eight items. Example items on the scale are ‘I think that I could easily become attached to another organisation as I am to this one’ (affective commitment); ‘I do not feel a strong sense of belonging to my organisation’ (continuance commitment) and ‘It would be too costly for me to leave my organisation right now’ (normative commitment). The reliability estimates of this scale are found by Meyer and Allen (1997) to have internal consistencies with the dimensions varying between 0.73 for normative, 0.79 for continuance and 0.85 for affective. The overall reliability estimates exceed 0.79 (Meyer & Allen, 1997).

Turnover intention

The Turnover Intention Questionnaire, developed by Roodt (2004), was used to measure turnover intention. This scale consists of 14 items. The items are measured on a seven-point intensity response scale anchored at extreme poles (e.g. 1 = ‘never/to no extent/low/always’ – low intensity to 7 = ‘most of the times/to a large extent/high/always’ – high intensity). Example items on this scale include ‘How often have you considered leaving your current job?’ Du Plooy and Roodt (2013) reported a Cronbach’s alpha score of 0.80 for their study in the Information and communication Technology sector. Two earlier studies (Jacobs, 2005; Martin, 2007) proved Roodt’s (2004) questionnaire to be both reliable (a = 0.913 and a = 0.895, respectively).

Data analysis

The reliability of the questionnaires was ascertained through the reliability analysis procedure of Statistical Package for Social Sciences version 25. Thereafter the total scores for use in the correlation and regression analyses were calculated. Both the Pearson correlation and linear regression analyses were subsequently performed to confirm or reject the hypotheses formulated in the present study.

Correlation and regression

Pearson correlations were performed on the data to determine the nature of the bivariate relationships among the variables used in the study. In addition, the correlations provided preliminary inputs for regression analyses as correlations among variables are assumed before performing a regression analysis. Table 1 presents the Pearson correlation results between POS, organisational commitment and turnover intention.

Cohen’s (1988) guidelines were used to interpret the Pearson correlational output. According to Cohen (1988), r coefficients between 0.10 and 0.29 indicate a small correlation; between 0.30 and 0.49 denote a medium correlation and between 0.50 and 1 indicate a large correlation. Significant large correlations were found among POS, organisational commitment and turnover intention (see Table 1). A strong negative relationship was found between turnover intention and POS (r = −0.745; p < 0.01). A strong negative relationship was found between organisational commitment and turnover intention (r = −0.590; p < 0.01), while a positive and strong relationship was observed between POS and organisational commitment (r = 0.599; p < 0.01).

The next step after the Pearson correlational analysis was to determine the extent to which organisational commitment and POS predict turnover intention. This was achieved through the use of linear multiple regression analysis. Table 2 depicts the model summary results. The R value of 0.766 indicates a high degree of correlation, while the R² or adjusted R² value indicates how much of the total variance in turnover intention is explained by the model. It shows that the model explains 58.7% of the variance in turnover intention.

The analysis of variance (ANOVA), as reported in Table 3, tests the null hypothesis that multiple R in the population equals 0, and reports how well the regression equation fits the data (i.e. predicts the dependent variable) (Pallant, 2010). The regression model is statistically significant at p = 0.000 (p < 0.001), indicating that the model significantly predicts turnover intention (see Table 3).

The next step was to determine whether organisational commitment or POS determines turnover intention. Tolerance and variance inflation factor (VIF) values indicate whether the variables are correlating too high (above r = 0.90) hinting at multicollinearity. Tolerance values less than 0.10 indicate that the correlation among the variables under study is high, suggesting the possibility of multicollinearity. The VIF values above 10 would indicate multicollinearity. Table 4 indicates that both the tolerance and VIF values are within the acceptable range.

The significance column in Table 4 indicates that both organisational commitment and POS are making a statistically significant and unique contribution to turnover intention. Perceived organisational support accounts for 61.1% of the variance (b = −0.611; t = −8.616; p < 0.001), while organisational commitment explains 31.46% (b = −0.223; t = 3.146; p < 0.001).

Ethical considerations

Before conducting the present study, ethical clearance was sought from the University of the Western Cape Ethics Committee, Registration no: 14/3/27. This was also subject to receiving permission from the aviation organisation used in the study. Participants were informed of the purpose of the study as well as their rights to anonymity, voluntary participation, confidentiality and informed consent through a cover letter provided as part of the questionnaire.

Discussion

Perceived organisational support is grounded in the assumption that to meet and access socio-emotional and tangible benefits, employees must perceive that the organisation values their contributions, their work effort and their general well-being (Eisenberger et al., 1986, 2001). For employees, socio-emotional resources and tangible benefits come in the form of respect, caring, fair wages and benefits (Eisenberger et al., 1986, 2001). Organisational commitment is grounded in the assumption that individuals, who identify with the organisation and strongly believe in their organisation’s goals and objectives, will exert considerable effort on behalf of the organisation and desire to remain a member of the organisation (Hussain & Asif, 2012). The results of this study support Meyer and Allen’s organisational commitment model and partially support Eisenberger’s (1986) Perceived Organisational Support Theory.

Hypothesis 1

This hypothesis was confirmed as findings reveal that when POS increases, it seems to have an inverse relationship with employees’ intention to leave the organisation. This implies that as employees experience socio-emotional resources and tangible benefits such as respect, caring, fair wages and benefits, their intention to leave the organisation is reduced. This finding is consistent with Eisenberger et al.’s (1986) Perceived Organisational Support Theory, as well as various other theorists who found a strong inverse relationship between POS and turnover intention (Al-Sakarnah & Alhaway, 2009; Batt & Colvin, 2011; Cho, Johnson, & Guchait, 2009; Islam et al., 2013; Nasyira, Othman, & Ghazali, 2014).

It is essential for employees to feel valued and cared for by the organisation. Cho et al. (2009) identified that those who are not cared for and valued by the organisation are often less committed and are more likely to leave the organisation. Employees who do perceive a climate of support will also tend to experience higher levels of commitment, as well as fewer intention to leave the organisation (Batt & Colvin, 2011). Furthermore, regardless of whether POS has a direct or indirect effect on turnover intention, researchers suggest that POS could possibly lead to feel obligations towards the organisation (Meyer & Allen, 1991; Tekleab, Takeuchi, & Taylor, 2005) and motivate employees to remain with the organisation leading to lower turnover intention (Meyer & Allen, 1991).

Hypothesis 2

This hypothesis was confirmed, as a strong inverse statistically significant relationship between organisational commitment and turnover intention was established. The results of this study suggest that employees are less likely to leave the organisation because of the costs associated with leaving the organisation, the emotional bond with the organisation and their moral obligation felt towards the organisation (Allen et al., 2003; Meyer, Stanley, Herscovitch, & Toplyntsky, 2002).

Hypothesis 3

This hypothesis was also confirmed. A strong positive statistically significant relationship was established. The results of this study suggest that when employees feel that their organisation values their contribution and cares about their well-being, they will express higher levels of affective, normative and continuance organisational commitment. A study conducted among 302 academic staff members from a South African higher learning institution also revealed a positive relationship between POS and employee organisational commitment. The researchers are of the opinion that as employees perceive enough support from their employer, they become more committed to the organisation (Batt & Colvin, 2011; Chuebang & Boatham, 2011; Mabasa et al., 2016; Ucar & Otten, 2010).

Hypothesis 4

The fourth hypothesis examined whether POS or organisational commitment predicted turnover intention. Results of this study found that POS and organisational commitment predict turnover intention. Dawley et al. (2010) also found POS to be a predictor of turnover intention. Their findings identified that when organisational leaders demonstrated that they care for their employees, turnover was minimal. Similar to the findings of the current study, Perryer et al. (2010) found that both organisational commitment and POS were significant predictors of turnover intention. They found that employees with low levels of commitment, but high levels of support from the organisation, are less likely to leave the organisation.

Limitations and recommendations for future research

Firstly, although all employees at the selected company in the aviation industry were invited to participate in this research (N = 240), only 144 surveys were received but 131 questionnaires were valid, thereby representing a 55% response rate. Secondly, the sample in this study included international employees with English not necessarily being their first language, which could have been a contributing factor to the low response rate. A larger sample size would have increased the strength of the research results. Thirdly, the current study made use of self-reported questionnaires that are susceptible to social desirability, thereby introducing biases and misrepresentations such as under-reporting behavioural tendencies. Fourthly, collecting data at a single point in time does not address maturational effects; longitudinal studies may resolve this problem (Terre Blanche & Durrheim, 1999). Furthermore, the use of confirmatory factor analysis might have helped control for measurement error. The sample size, however, was not big enough for structural equation modelling as most softwares require samples bigger than 200.

Managerial implications

Several studies have predicted the extent to which turnover intentions are associated with the actual staff turnover (Dewettinck & Van Ameijde, 2007; Lambert, 2001; Steel & Lounsbury, 2009). It is therefore important to identify the work and psychological determinants of turnover intention to be proactive in its prevention and containment (Van Schalkwyk, Du Toit, Bothma, & Rothmann, 2010). The findings of this study suggest that employees from the selected organisation in the aviation industry are likely to have strong turnover intention when they feel that they are not getting some support from their organisation, which results in negative implications on their organisational commitment.

Organisational comprehension

Organisational comprehension refers to how well employees understand the organisation, including its strategic direction, social dynamics and physical layout (Bauer, 2007; Cooper-Thomas & Anderson, 2005). This awareness, according to Bagraim (2010), is a necessary prerequisite to develop affective commitment in employees because it is difficult to identify with, or feel loyal to, something that you do not know very well. Furthermore, lack of information produces uncertainty, and the resulting stress can distance employees from that source of uncertainty (e.g. the organisation).

Managerial/Supervisory support

The quality of the relationship between employees and their line managers has significant and important undertones on the attraction and allocation of POS (Rhoades & Eisenberger, 2002). Employees value opportunities that allow them to exercise their discretion in decision-making, and partake in privileged communication and positive recognition of their work through praise or positive comments (Wayne et al., 1997). By offering positive feedback and providing advice, line managers help instil confidence in their employees. Therefore, supervisors should develop a meaningful relationship between themselves and the employee. In summary, leaders and managers have an important role as they can positively influence employees by giving support and vision, and make employees feel that they are important to the organisation.

To help cultivate a positive organisational commitment attitude among employees, several organisations have introduced employee support programmes (Grant, Dutton, & Roso, 2008). These are formalised interventions geared towards providing employees with the necessary psychological, emotional, financial and instrumental assistance over and above the traditional pay, benefit, recognition, and training and development programmes offered by most human resources departments. The employee support programmes usually include some assistance with work–family programmes (e.g. child and elderly care), among other various forms of help and aid (Cascio, 2003). In addition to employee support programmes, other interventions, such as corporate volunteer programmes and corporate social responsibility initiatives, would also propagate some affective commitment (Grant et al., 2008).

Conclusion

This research aimed to add to the existing body of knowledge on the constructs of organisational support, organisational commitment and turnover intention in the aviation industry. In this regard, several suggestions have been made so that the aviation organisations may begin to implement programmes aimed at increasing levels of organisational support and organisational commitment for their employees as this will also promote passenger safety. As employees have been leaving the organisation increasingly over the period of 2012–2014, the organisation would benefit from evaluating possible reasons other than perceived support and organisational commitment, which could account for this turnover and take into consideration the recommendations put forth for the organisation. Given the high rate of turnover in the organisation under investigation, cultivating organisational commitment through organisational support is likely to increase stability in the aviation industry, especially in view of the increase in terrorism and passenger safety concerns.

Acknowledgements

This article is based on the dissertation by one of the authors available at https://core.ac.uk/download/pdf/58915288.pdf.

The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.

M.S. was responsible for the conceptual write-up of the article and data collection. R.J. was the project leader responsible for the write-up of the article. B.M. was responsible for the statistical analysis and write-up of the article.

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

Data sharing is not applicable to this article as no new data were created or analysed in this study.

The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of any affiliated agency of the authors

References