Using social capital to develop South African principals and schools

One feature of deprived communities is a lack of social capital. Yet, research reports that social capital contributes to poverty reduction and positively impacts schools and student educational outcomes. In South Africa, there is a deficit in social capital in under-resourced and underperforming schools that limits students’ educational opportunities and achievement. Partners for Possibility (PfP) responds to the lack of social capital in South African schools by partnering school principals and business leaders to develop support structures such as collaboration, networking, and professional learning communities. Findings from a site visit, conversational interviews, and examining participants’ portfolios indicate that PfP provides opportunities for developing three types of social capital: structural, cognitive, and relational. These produce options that would otherwise be unavailable to these students. The discussion raises issues about social capital as a resource for development and offers suggestions for further research.


3
people and communities are granted access to social capital, they gain power through people and institutions who have influence (Woolcock & Narayan, 2000). In turn, this spurs social, economic, political, and educational change.
Students in poverty are disadvantaged by a social-capital gap that negatively impacts educational outcomes and achievement as well as school attendance and that can increase student dropout rates and grade repetition (Fleisch, 2008;Spaull, 2015). However, empirical evidence demonstrates that social capital contributes to poverty reduction and positively impacts schools and student educational outcomes (Lindfors et al., 2018). Accessing social capital for poor schools is difficult. Taliaferro and Flood (2014) suggest that it is essential for principals to know how to build and leverage social capital to create opportunities for their schools, communities, and students. This is the case in South Africa, where a deficit in social capital in underperforming schools limits students' educational opportunity and achievement (Fleisch, 2008).
Responding to this deficit is Partners for Possibility (PfP), a program that addresses the social-capital deficit in South African schools by providing development and support for principals and schools. By partnering school principals with business leaders to develop collaboration, networking, and professional learning communities, PfP supports principals' work and improves under-resourced and underperforming South African schools (Desert 2 Desert, 2018).
Before beginning this discussion, a brief caveat regarding the researchers' positionality is necessary. In March 2019, on behalf of the World Innovation Summit for Education (WISE), I traveled to South Africa to examine the PfP program. According to Holmes (2020, p. 2), positionality is defined by locating the researcher within three areas: "(1) the subject under investigation, (2) the research participants, and (3) the research context and process". For this study, I knew little regarding the PfP program other than that it was the recipient of the 2018 WISE Award as a successful and innovative project addressing global educational challenges. Before the site visit, I read PfP documents, Collins (2015) on PfP, and academic work completed on South African education and schools.
From my academic position, which evokes critical social theory, I entered with a predetermined position regarding an apprehensive involvement of business and industries in schools. The concern in this school-business relationship is that education should not fulfill the plan of capital and enterprises. Instead, education should cultivate learning that promotes individuality, citizenship and community, social justice, and the strengthening of democratic participation. Furthermore, growing up as a "poor" white working-class kid also played a role in how I viewed business leaders' investments and donations from privileged positions and community improvement. Therefore, I approached the context as an academic who examined how an award-winning program impacted South African schools. This position guided the program's analysis and spawned the use of social capital as a framework to understand PfP. Finally, I have no commitment or vested interest in PfP.
In what follows, the concept of social capital is developed, followed by a brief background of education in South Africa. Next, PfP, the flagship program of registered non-profit Symphonia for South Africa (SSA), is developed. This is followed by presenting the site visit findings and discussing how business leaders and principals use social capital to develop underfunded and underdeveloped schools in South Africa.

Social capital
Social capital is an intangible resource developed through social relations (Plagens, 2011). Bourdieu (1986, p. 21) describes social capital as the aggregate of the actual or potential resources which are linked to possession of a durable network of more or less institutionalized relationships of mutual acquaintance and recognition-or in other words, to membership in a which provides each of its members with the backing of the collectively-owned capital, a "credential" which entitles them to credit, in the various senses of the word.
The essence of social capital is not the network structure itself but rather the potential actual tangible products or resources within the network that individuals, groups, and communities can attain by forming these relationships (Bourdieu, 1986;Coleman, 1988). Lin (1999) suggests that social capital contains three main elements: resources are (1) embedded in a social context, (2) accessed or mobilized, and (3) used in purposive action. These resources include skills, knowledge, expertise, or information beyond one's immediate network but that, when accessed, enable one to gain advantages. For people and communities, gaining access to social capital allows them to gain power through individuals and institutions who have influence (Woolcock & Narayan, 2000). Claridge (2018) suggests that social capital contains individual, personal, and collective or public dimensions. Private social capital centers on individual relationships through which a person can develop networks and social capital and reap exclusively private benefits (Cloete, 2014) such as a job opportunity. Public social capital moves beyond the individual, benefiting the community and groups of people beyond those who possess it (Claridge, 2017), such as an individual securing service for a school donated by a business. Public social capital can benefit and empower the wider community and the collective whole (Muijs et al., 2010;Thomas, 1996).

Social capital and education
The influence of social capital on students' educational achievements has been established (Pishghadam & Zabihi, 2011) and might explain the disparities in academic success and why some schools and students outperform others (Plagens, 2011;Rogošić & Baranović, 2016). Social capital influences educational attainment, such as high-school graduation and college enrollment (Carbonaro, 1998;Muller & Ellison, 2001), grades, and test scores (Hao & Bonstead-Bruns, 1998), and students' educational outcomes (Israel et al., 2001;Muller 2001). Perna and Titus (2005) argue that increased social capital enables parents to develop positive academic patterns such as expectations and obligations and become more confident in advocating for their children.
Principals can use social capital to provide students opportunities that they lack because of their limited access to social-capital networks (Taliaferro & Flood, 2014). Principals are instrumental in delivering students with the benefits of social capital that raise students' achievement outcomes (Rodrigues & Child, 2012). Taliaferro and Flood (2014) offer strategies for principals to increase social capital. These include developing interpersonal relationships that lead to increased social capital, joining well-connected 1 3 network groups to access benefits, developing mentoring relationships, and identifying and tapping into current relationships.

Analytical framework
The analytical frame used in the study is adapted from Claridge (2017) using the three dimensions of social capital and the two types of social capital-individual and collective (see Table 1). Claridge (2017) argues that a general agreement is that social capital is difficult to measure with high validity. However, social capital can be inferred from its dimensions, indicators, levels, characteristics, and whether it is individual or collective. Claridge (2018) argues that structural social capital is what an individual knows that makes up a network which can be called upon for benefits such as information, assistance, or expertise. The structural dimension contains the network's social system, including arrangement, hierarchy, presence or absence of network ties, and density and connectivity of an individual's network (Nahapiet & Ghosal, 1998;Muniady et al., 2015). Structural social capital facilitates conditions of accessibility for mutually beneficial collective actions and benefits such as obtaining information or accessing resources, but these opportunities are limited to significant individuals (Claridge, 2018).

Cognitive social capital
Cognitive social capital refers to shared representations, interpretations, and meaning systems within relationships or networks (Nahapiet & Ghosal, 1998). The cognitive dimension includes shared systems of meanings, values, attitudes, beliefs, and a shared culture that predisposes people toward mutually beneficial collective action (Claridge, 2018;Nahapiet & Ghosal, 1998). When individuals share a purpose and genuinely care about the community, they are more likely to work together (Plagens, 2011). Gooderham (2007) points out that cultural and economic distance play a role in forming cognitive social capital. The greater these distances, the weaker the degree of cognitive social capital. Sustained interaction is necessary to develop and maintain this dimension of social capital.

Relational social capital
Relational social capital includes the resources created through an individual's interactive relationships (Al-Tabbaa & Ankrah, 2016). The qualities and characteristics of personal relationships are the essences of relational social capital. These include trust, obligations, expectations, respect, and even friendship (Gooderham, 2007). An essential element of relational social capital is associability-the willingness to subordinate individual goals to collective goals (Lazarova & Taylor, 2009

Education in South Africa
To grasp the current conditions of education in South Africa, it is vital to consider the economic and social impact of apartheid. During apartheid, segregation was an ideology and practice that legitimized education and economic inequalities (Beinart & Dubow, 1995). The 1953 Bantu Education Act established a system that denied education to most of South Africa while others were educated in a system that prepared Black and non-white students for racial stratification of unskilled occupations (Collins, 2015;Gradín, 2013). This legislation resulted in Black students being allotted about onefifth of the school funding of their white peers (The Economist, 2017). The educational inequalities resulting from the Bantu Education Act were so vast that white children received a free and compulsory education equivalent to Western countries while Black, colored, and Indian schools were not mandatory or free and lacked necessary provisions as electricity plumbing (Collins, 2015). It is unreasonable to measure the devastating effects apartheid has on families and children.
In 1994, with the termination of apartheid and the first democratic elections, equitable education became a national priority. The South Africa Department of Education set a mission to provide equal education and implemented plans to change education from its apartheid past (Ngcobo & Tikly, 2008). For example, South Africa spends 6.4 percent of GDP on education compared to EU countries' average spending of 4.5 percent (The Economist, 2017). Still, there are considerable discrepancies between expenditures and outcomes (Mlachila & Moeletsi, 2019). However, with years of financial investment and reform legislation, South Africa has been unsuccessful in raising historically disadvantaged learners' performance (Ngcobo & Tikly, 2008). Currently, roughly 20,000 of the 25,000 schools in South Africa are deemed as failing (Collins, 2015), and only 37 percent of children starting school passing the matriculation exam; just 4 percent earn a degree (The Economist, 2017, para 2). Spaull (2015) argues that even though racial segregation was abolished in South Africa, schools where primarily white students attended under apartheid remained functional while schools for Black students remained dysfunctional and incapable of effectively teaching the necessary and literacy skills.
When it comes to South African principals, there are no rigorous criteria for educators to be appointed as school principals (Bush et al., 2011;Townsend & MacBeath, 2011). Most principals are educated as teachers (Van der Berg et al., 2011) and are appointed without leadership and management qualifications or experience, unprepared for their roles and lacking in essential leadership and management skills (Bush & Oduro, 2006;Heystek, 2016;Mestry, 2017). Botha (2004) posits that principals must provide instructional leadership, including curriculum supervision, improvement of the instruction, and development of community relationships. They also must manage the budget, the school buildings and grounds, and fulfill educational policies and acts. Furthermore, most South African schools fail to have a critical system for effectively managing physical or educational resources (Mestry, 2017).
According to research by Bush and Heystek (2006) and Piggot-Irvine et al. (2013), South African principals require the development of support networks, policy issues, interpersonal skills, and to improve their role in administrative, financial, and humanresource management. Several types of support would be beneficial to meet these principals' needs, yet they are often unavailable. These include collaboration, professional learning communities, and networking.

Partners for possibility
Partners for Possibility (PfP) aims to improve South Africa's schools by developing school principals. PfP centers on under-resourced and underperforming South African schools and works upon the premise that the crucial difference between an effective school and an ineffective one is leadership. If developed, principals can have a potentially significant and long-lasting impact in their schools. The program's leadership approach is evident in its philosophy: "a transformational leadership development experience to develop leaders who collaborate effectively for the benefits of their organizations and communities" (Symphonia for South Africa, 2017, p. 3). The program's foundation is the development of mentoring relationships between principals and experienced business leaders. It enables principals to build a more well-rounded skill set and provides principals with opportunities to develop their social capital by giving them access to networks that may have been absent (Taliaferro & Flood, 2014).
The PfP program is one year in duration and contains several components, requiring participants to commit a minimum of 150 hours, or 15 hours per month (Symphonia for South Africa, 2017). The majority of learning (70 percent) takes place within the school community, along with learning through networking, collaboration, and developmental relationships (20 percent), and formal training coursework (10 percent). Table 2 provides the program structure, hours required, and component descriptions: Involvement in PfP is available to principals and business leaders throughout South Africa, and joining the program is on a volunteer basis. PfP uses a cluster structure, where partnerships are grouped together to develop a learning community. These clusters meet "regularly to share insights, discuss challenges, and provide mutual support. Each group is supported by a Learning Process Facilitator (LPF) who is a professional coach" (Desert 2 Desert, 2018, para. 5). Participants are taught to reflect and to think about the complexity and challenges of school leadership in their context.
Participants engage in reflection on the complex challenges that face these school leaders in their unique context. To date, 1,282 principals across South Africa have joined the program, with a 95 percent completion rate. (For additional details and updated numbers about PfP, see www. pfp4sa. org).

Methodology
This study uses a qualitative exploratory case-study method to explore a phenomenon within its real-life context using various data sources. It enables the researcher to gain an in-depth holistic view of the context allowing for a description, understanding, and explanation of the situation (Baxter & Jack, 2008). The research objectives for the case study are as follows: • To explore the PfP program from the perspectives of principals, business leaders, and others directly involved in the program. • To examine how social capital provides development and opportunities for schools and students. Data collection included conversational interviews and document analysis. Conversational interviews are unstructured interviews based on an unplanned set of questions, where questions emerge based on the specific context (Gray, 2009). The conversational interview produces data through informal discussions of relevant topics with participants resembling more of a conversation than an interview (Neuman, 2014). It is challenging to audiotape conversational interviews. Therefore, copious field notes were taken during all conversations. At the end of every interview, there was time allotted to make immediate field notes. Each participant was required to complete a portfolio after the program. These portfolios were examined.
The researcher requested to meet with principals, business leaders, and others who could provide insight into the program. The staff of PfP selected the participants. Conversational interviews were conducted in several different contexts, such as during tours of schools and meetings with principals and business leaders in schools and business places. PfP leadership often engaged in conversations with the researcher in their offices, lunches, and car rides to various sites.
There were 24 participants in this study. These are as follows: the founder and cofounder of PfP, the regional manager for PfP in the Western Cape, five PfP principals, six PfP business leaders, the director and one member of the PfP Monitoring and Evaluation team, a facilitator for Time to Think, five LPFs, one Flawless Consulting Facilitator, and one supervisory coach.
A preliminary thematic analysis was conducted to assign codes to understand the content. The analysis results provided various patterns and themes identified, reviewed, and titled. Quotes were added to a particular theme.

Structural social capital
The structural dimension of social capital includes networks. Findings indicated that principals' enrollment in PfP and partnership with a business leader increased their networks' density and connectivity. This enabled principals to develop solutions to many of the problems unique to their schools. PfP staff members, principals, and business leaders described and offered examples of networks' effectiveness outside the education sector. Principals made their needs and the needs of the schools and students known through collaboration with business leaders. These were eventually communicated to others within the business leaders' network. It was clear that the principals were learning to build a network that provided access to resources benefiting both the individual and the collective good.
For example, a principal explained that when he first arrived at the school, the school's conditions were deplorable. The community and school climate reflected hopelessness, primarily because of the scarcity of government finances and the school and community's lack of capacity to solve many pressing problems. Based on a lack of support and a sense of being alone to solve these problems, the principal voluntarily joined PfP. With his partnered business leader's assistance, the faculty, staff, and students slowly saw transformations in the school. For example, there was no fence around the school, allowing gang members to roam the school grounds freely. Accessing social capital via his business partner, a company that did work with his business partner built a fence at the school free of cost. With the new fence, gang members were prevented from entering the school, providing security for the students and the community. Based on research, one can infer that providing a secure learning environment aids in learning since research demonstrates that school safety is an essential condition for learning (Cornell & Mayer, 2010).
During the year in the PfP program, the principal and business leader's partnership developed, and networking led to building a library not only for students but also for public use, developing a large garden to grow food for the school and community, and the hiring of a gardener. Since the previous principal could not address these school needs, this principal suggested that the teachers, staff, and community upon seeing these developments, grew confident in him as a principal and were more willing to support the school's leadership. These changes resulted in individuals in the community helping at the school to work and support the school. There were abundant examples of social capital and networking's collective good through site visits and portfolios, resulting in school development. For example, a newly constructed school hall allowed faculty and parent meetings to be inside instead of outside. Principals reported receiving new technologies and developing well-equipped math and science labs. At one school, gravely needed additional reading and math programs were provided to students and communities, and other classes, such as parenting classes, were offered to the community. Basic internet connectivity became a reality for some schools, while others received iPads and other IT teaching and learning tools.
Various new equipment such as sports equipment and musical instruments were provided to some schools, and vision and hearing testing equipment were added to schools needing these technologies. Finally, in one school students developed fundraising and budgeting skills and eventually traveled on an educational trip to the Netherlands.
The above examples demonstrate the products and benefits of social capital as they emerge in several partnerships, all adding educational and social value. It is essential to mention that PfP does not require any transfer of material resources. These partnerships were not defined solely by structural capital. Evidence demonstrated that cognitive social capital existed and was developed through the partnership. More importantly, not all schools experienced the level of growth expressed above since each had a unique context.

Cognitive social capital
An essential aspect of cognitive social capital is the shared meanings and beliefs that move people toward reciprocally valuable collective action. Cognitive social capital comes from relationships where individuals have a sense of social responsibility. The idea is that when individuals share a purpose they are more likely to work together. The development of cognitive social capital is evident when conversations with principals and business leaders revealed a shared belief that involvement in schools and communities was a social responsibility that could make a difference in South African communities.
Cognitive capital was evident when several principals and business leaders viewed PfP as a vehicle to impact and improve education directly. For example, one business leader stated that he was "giving back to the community, which is his social responsibility … these students are the future workforce for the country and my employees". Another business leader suggested that businesses are not just about making a profit but that "giving back to the community" should be a vital purpose of companies.
There is reciprocity evident in these relationships. Most of these business leaders had been isolated from disadvantaged, non-white communities, and their work with principals was their first interaction with these communities. Many were unaware of the conditions and struggles of those still feeling the lingering effects of apartheid. They were able to develop a shared understanding of principals and communities. One business leader wrote an excerpt in his portfolio that he "gained insight into the social issues which drive the major dysfunctions in South African society". His reflection included a statement that illustrated how his perspective had changed, and he began to develop an understanding of education and schools through the eyes of the principals. He wrote, "in the bigger scheme of things, there are always people to grapple with much bigger issues that I could never imagine and that there are always people worse off than me and that I need to appreciate more". Another business leader suggested that her involvement with the principal and school exposed her to the living, learning, and economic conditions of the "other", to students and neighborhoods that she had been isolated from for a decade, in the process developing self-awareness and acknowledging her responsibility to others. Another business leader expressed how her perspective had changed, stating that "by walking with a disadvantaged school and community for a year, you see real issues that result in the dysfunction of many children and how this contributes to issues South Africa grapples with, such as unemployment". This experience is "opening the eyes of people with power", and through the changing of their perspectives via networking, schools are changing too.
Principals developed a shared narrative with business leaders by embracing and creating a "networking perspective", seeing possible support for their schools beyond the educational sector. For example, one principal wrote in his portfolio, the powerful key that this program holds is applying business thinking to solve social problems …. To scale solutions, we need to generate profit; it is when we can generate additional income or profit that we can start acquiring more resources to scale the solutions we generate. This is the benefit that business thinking brings the program.
Often principals fail to consider perspectives outside education and how others approach problems and solutions. For example, a principal showed this new understanding when he stated that instead of thinking of the community as "out there", he started to view the community, community members, and businesses as potential ways of approaching and solving the school's issues.

Relational social capital
Relational social capital centers on the resources accessed through an individual's relationships, such as expectations, respect, and friendship. One resource secured by principals from their relationships with business leaders was professional support that reduced the isolation felt by most of these principals. Several aspects of relational social capital were evident in the principal and business leaders' partnerships, and a sense of support, trust, respect, and friendship emerged from the principal and business leader partnerships.
From conversations with principals and business leaders, it was evident that an important motive for why principals became involved with PfP was to secure professional support. All principals and some business leaders indicated that they felt isolated in their current positions. One principal stated, "the loneliest jobs are a CEO and principal". Principals and business leaders told of various times they needed support and how their relationship with business leaders provided that support as they faced complex challenges within their schools. For instance, two principals explained how the partnership and regular meetings provided them with a sense that they were not alone in their schools. One stated: "Meeting with my partner provided me with confidence that I was not alone and that I could solve challenges with others' assistance".
Another way in which relational social capital reduced principals' isolation was with the Community of Practice (CoP) meetings, the non-compulsory professional learning communities, which are open to all partners. The CoPs include eight to ten pairs of partners, guided by an LPF, and are a safe space to share their thoughts and challenges. It was clear from discussions and examples that friendship and trust developed between principals and business leaders and within the group itself. The discussion centered on leadership concerning challenges facing members and issues dealing with organizational change. Principals explained difficult situations in their lives and problems in their schools that they could not discuss with anyone at school but could address in the CoP meetings. One principal stated that CoP allowed "him to grow as a person and begin to see educational challenges and problems not as black and white as he did before . . . all in a supportive environment".
Over time, the partnerships and the CoP groups developed a commitment to each other-an aspect of relational social capital. Participants in this study often expressed that the CoP helped them feel less isolated, and they were not alone in developing and leading the school. It is important to note that, as pointed out by several PfP staff members, the quality and depth of relationships vary for each principal-business partnership. Field (2003) posits that social capital is an asset that people can use to improve communities and knit the social fabric through interaction. PfP clearly understands this because the program provides principals, business leaders, and communities with opportunities to use social capital so they can work together and develop their communities and schools.

Discussion
Several issues should be discussed. First, the South African government rates businesses under the Broad-Based Black Economic Empowerment (BBBEE) system for full disclosure. This program is designed to rectify the inequalities of apartheid. Companies are rated on five elements (ownership, management control, skills development, enterprise development, and socio-economic development), and, based on their rating, tax incentives are implemented. The BBBEE system's element of socioeconomic development, or business spending on assisting charitable organizations (Friedman, 2017), does not take away from those involved in PfP but illustrates how the government could play a role in encouraging businesses to develop communities.
Second, there is a concern regarding business partnering with education. In this context, business leaders provide access to networks and often offer different leadership approaches, problem-solving, and change. However, business leaders have little to offer in instructional leadership and other dimensions of education. The educational needs of principals need to be provided within different aspects of the program. The CoPs or principals may need to seek other avenues for this type of professional development.
Third, awareness is needed that business and education are still different. The view held by many is that schools are the problem and business is the solution. That may have some issues, but principals must integrate business solutions into schools and the appropriate areas, keeping some dimensions of schools such as the curriculum within the education sector.
Another issue that merits discussion is PfP's approach to school development. South African schools and principals have a long history of top-down policies. PfP's bottom-up approach to change reflects the critical pedagogy of Paulo Freire (1972). Freire was critical of institutional domination and oppression by imposing top-down strategies. Too often, change is forced on schools with little consideration for their cultural fit. Furthermore, Freire argues that top-down programs deprive people of their agency to improve their conditions, and humans, through their work, can change the world. PfP utilizes a bottomup approach encouraging challenges and solutions to emerge from individuals embedded in the school's particular context rather than being imposed by outsiders reflecting Freire's philosophy. PfP uses community-building skills, mobilizes parental and community involvement in their schools, and allows communities and individuals to act independently and make free choices.
However, there is a dichotomy between PfP's approach and intra-community improvement. Often business leaders invest from the outside from a privileged position. This raises external, top-down initiatives that center on fixing the situation and concern about sustainability and community reliance on outside sources. Marais (2012) points out that the links of top-down initiatives meeting bottom-up development are vital for social capital. This requires incremental development of links to the broader community, knowing that a community's development of "social capital in the form of integration can be the basis for launching development initiatives, but it must be complemented over time by the construction of new forms of social capital, i.e., linkages to non-community members" (Woolcock, 1998, p. 175). It is, therefore, vital that all stakeholders recognize the need to connect "topdown resources and bottom-up capacity building" (Woolcock, 1998, p. 179).

Further research and limitations of this study
This initial study and findings are not conclusive but could initiate further research. Research studies that examine the impact of financial investments in buildings, resources, and equipment to assess learners' benefits would be worthwhile. Inferences can be made about the effects of PfP on students' learning and outcomes, but further research is needed to empirically document the impact on student learning and outcomes when appropriate.
There were several limitations to this study. The limited time spent on site and the low number of participants are limitations. Furthermore, the concern for bias in selecting the participants should be considered a limitation of this study. Future studies must include additional research sites, and sampling a broader population will provide valuable insights. Nevertheless, the initial research offers valuable insight into PfP and how social capital can develop principals and schools.

Conclusion
In closing, the knowledge gained from this study is not so much based on the evaluation of the PfP program but rather the importance of social capital and how it was and can be used to improve schools. PfP does not provide a one-size-fits-all blueprint for school development. The opportunities provided by accessing the three types of social capital differ. These differences depend on the school's needs, the relationship between the principal and business partner, and other contextual factors. However, social capital is necessary to resolve social problems at micro and macro levels and to form bridges between local economic development and empowerment approaches (Woolcock, 1998;Kleine, 2007). Viewing schools through the lens of social capital draws attention to the numerous benefits obtained through networking, leading to cooperation, and, in this context, the improvement of schools and communities.
Funding Open Access funding provided by the Qatar National Library.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.