Structure in knowledge sharing networks The importance of homophilous behaviour was identi- ﬁed by Creswick and Westbrook30 who examined how a network of staff in an Australian hospital renal ward sought medication advice. Most communication occurred within professional groups. Similarly, a study of a network of staff in an Australian hospital emergency department by Creswick et al31 found that, when seeking advice, individuals were more closely connected to colleagues in their own professional groups. Heng et al26 used SNA to explore the brokerage role of facilities management in an Australian hospital. Facilities managers actively bridged information and knowledge across different functions, ﬁlling structural holes within a communication network structure and being positioned to identify inter-disciplinary opportunities. Although various studies discussed above identiﬁed features of network structure associated with better performance, Rangachari provided one of the few to link network structure with quality outcomes.28 This research in four large US teaching hospitals used SNA to explore the relationship between the organisational knowledge sharing structure related to quality, and hospital coding performance related to quality. Good-coding perfor- mance was associated with a knowledge sharing network structure rich in brokerage and hierarchy (with senior leaders coordinating knowledge exchange related to quality and connecting the organisation with the external environment), rather than density (with everyone connected to everyone else). DISCUSSION This review examined the empirical research on the structure of networks of health professionals, with regard to the effectiveness and sustainability of networks, espe- cially in relation to quality of care and patient safety. There is a convergence of many networks, independent of their age, function and scope, to similar architectures. This has allowed researchers from different disciplines to embrace network theory as a common paradigm.56 Consistent with this theory, a number of the reviewed studies demonstrate the link between features of network structure and outcomes. Quality-related performance is associated with a knowledge sharing network structure rich in brokerage and hierarchy, rather than density.28 Social inﬂuence in a network is positively related to physician performance,39 social capital predicts overall job satisfaction,48 positive social climate protects employees against burnout,49 and employees with more co-worker ties have lower organisational turnover.35 The evidence demonstrates that creating cohesive, collaborative networks (of professionals or agencies) can BMJ Qual Saf 2012;21:239e249. doi:10.1136/bmjqs-2011-000187 Systematic review pay dividends in coordinating care and attending to quality and safety issues and agendas.28 32 34 39 46 47 The presence of key players, often in management or lead- ership roles, who act as connectors to transmit infor- mation, bridge disparate groups, liaise across parts of networks and enable social and professional interaction is vital.26 27 32e34 They go by many names including mavens, connectors and bridges. In essence, they can facilitate communication and trust.27 However, the centrality of key players holding a network together can also be a vulnerability of such networks if they are relied on excessively and then leave, change roles or become marginalised.32 34 We have known for a long time that people naturally cluster together with those with whom they are comfortable: network theorists and sociologists call this homophily, and most people know this phenomenon as ‘birds of a feather ﬂock together’.29e31 Creating multi- disciplinary or interprofessional teamwork within and across networks is thus a challenge. To address this challenge, active bridge building between subgroups across commonly occurring organisational divides (professions, genders and generations) is very important in creating larger, more resilient professional networks.26 27 36 Strategies for quality improvement must address these factors, and the different characteristics of disciplinary networks (eg, nursing and medicine).27 As summarised in table 2, the characteristics of networks are important in facilitating greater levels of quality of care and patient safety. How far apart indi- viduals are, for example, their degrees of separation, how central the key actors are, how intense the network relationships are, how dense the network is, how hier- archical relationships are structured and how stable the network is and the actors within it are all factors that will determine how well functioning the network will be and how well the actions of interacting agents will cohere to provide services. The received wisdom from various studies seems to be that being in an effective network which encourages communication and facilitates trust helps people feel good about their relationships, and to reciprocate positively with others to develop social capital. Positively functioning networks are likely to contribute more broadly to an effective organisational culture and climate.49e52 57 In this respect, networks can represent not just the social glue of professional inter- action but the sociological building blocks of effective organisations. Finally, we should ask what is missing from the research evidence we have uncovered and discussed. Although a third of the studies link network structure with evidence of outcomes,28 35 38 39 47e50 most of the research examines only the structural features of health professional networks. Using multi-method approaches, 247