Mittman and colleagues [3] noted that health care professionals work within peer groups, which share common beliefs and assumptions and group norms, and that individual behaviour can be strongly influenced by these factors. They identified a number of strategies to facilitate the implementation of research findings by using these social influences. One strategy generating considerable interest is the use of opinion leaders.

Opinion leadership (more properly termed Informal Opinion Leadership; for ease of reading we refer to 'opinion leadership' throughout this article) is the degree to which an individual is able to influence other individuals' attitudes or overt behaviour informally, in a desired way with relative frequency [4]. This informal leadership is not a function of the individual's formal position or status in the system; it is earned and maintained by the individual's technical competence, social accessibility, and conformity to the system's norms. When compared to their peers, opinion leaders tend to be more exposed to all forms of external communication, have somewhat higher social status, and to be more innovative. However, the most striking feature of opinion leaders is their unique and influential position in their system's communication structure; they are at the centre of interpersonal communication networks – interconnected individuals who are linked by patterned flows of information.

There is only limited empirical evidence about the effectiveness of opinion leaders as health care change agents. Thomson and colleagues [5] identified only eight rigorous evaluations of opinion leaders in the health care literature. Six out of seven trials observed improvements in at least one process of care variable, although these results were only statistically and clinically important in two trials. One of three trials measuring patient outcomes observed an improvement that was of practical importance. They concluded that using local opinion leaders resulted in mixed effects and that further research was required before the widespread use of this intervention could be justified.

There are four approaches to the measurement of opinion leadership: sociometric methods, key informant methods, self-designating methods, and observation [4]. Sociometric methods [4,6] involve extensive analyses of leadership nominations within members of a peer group. Seven out of the eight opinion leader trials used a sociometric instrument developed by Hiss, [6] which seeks nominations for individuals who are knowledgeable, good communicators and have humanistic philosophies. Key informant methods ask a small(er) number of individuals, who are particularly knowledgeable about a network, to identify individuals who serve as main sources of information, influence or both. This method was used by the other trial. Self-designating methods [7] involve self-reporting, by all members of a network, of their own role as an opinion leader. This method has been used to identify individuals for marketing exercises and for studies promoting individual behaviour change; however, it has not be used to identify opinion leaders in health care professional groups. Observation methods involve direct observation and work best in small systems.

Although using opinion leaders to induce the rank-and-file to change behaviour has great intuitive appeal, we believe that a number of conditions are prerequisite to its use as an effective strategy. Firstly, there must be effective interpersonal communication networks. Secondly peer influence must work amongst professional groups. Thirdly, opinion leaders must be readily identifiable. And finally, the leaders must be inclined to adopt changes based on evidence, so that they can honestly influence others. Support for these four prerequisites is encouraging but not definitive. In some professional groups, it may be difficult to identify opinion leaders, or the group may be so diffuse that there are few opportunities for influence (un-cohesive or ineffective interpersonal networks). A further complicating factor is the uncertainty about whether – in any professional social network – there will be one set of all-purpose opinion leaders (polymorphism) or whether there are different opinion leaders for different issues (monomorphism).

Responses to the sociometric instruments demonstrated a wide variation across different professional groups and sites in the extent of nominating SOLs and the complexity of social networks [8]. These results suggest that the extent of social networks and potential coverage of the study population in primary and secondary care is highly idiosyncratic, and adequate coverage rates cannot be assumed. In contrast, relatively complex networks with good coverage rates were observed in both national specialty groups.

Both SOLs and SDOLs had characteristics of opinion leaders although the odds ratios and mean differences in continuous variables were higher in SOLs. Approximately one-third of generic SOLs also were nominated as condition-specific SOLs, and the condition-specific coverage rate of these SOLs was poor. Similarly, generic SDOLs were relatively unlikely to identify themselves as condition-specific SDOLs. These results suggest that opinion leaders are monomorphic, and that separate identification exercises would be needed for different conditions.

Case studies frequently identify the importance of individuals (opinion leaders, change agents, product champions) in leading and supporting change in the health service. However, these terms are not necessarily well defined, nor mutually exclusive. In this study there was poor agreement in the responses to the sociometric and self-designating instruments. SDOLs were relatively unlikely to have been identified as SOLs and vice versa. There are at least two possible interpretations of this. If the instruments are trying to identify the same construct of opinion leaders, one is performing poorly. Alternatively, the instruments may be identifying different constructs of opinion leaders. The sociometric instrument was rigorously developed [6] and has face validity, but remains the only instrument of its type and thus has not been validated against a comparable instrument. It emphasises opinion leaders who are knowledgeable, humanistic, and good communicators – characteristics identified by physicians as likely to influence their choice of educational influential (Table ​(Table1).1). Work in Norway [9] showed that general practitioners supported the concepts espoused in the sociometric instrument. The instrument demonstrates the extent of social networks and coverage of identified opinion leaders and has been successfully used to identify opinion leaders in randomised trials, which have demonstrated behaviour change. The self-designating instrument emphasises opinion leaders who are commonly consulted by colleagues and who give a lot of information (Table ​(Table2),2), and while the sociometric instrument may identify one construct of opinion leader, other types of leadership also may be influential (e.g., professional or academic leaders). However, there is scope for further exploration of the validity of the self-designating instrument within professional settings. These considerations highlight the potential conceptual and terminological confusion surrounding opinion leadership. Whilst this term is used in a specific technical way within the diffusions of innovation, marketing and social influence literatures, it is commonly used to describe any influential individual (educational, academic or political).

Response rates to the survey overall were moderate (57.8%). The response rate to the sociometric instrument was lower. During pilot work for this study, interviews with primary care respondents – after they had completed the instruments – suggested that they had some difficulties with the concept of opinion leaders, and the questionnaire was also seen as being rather abstract [8]. We have identified eleven studies that have used the sociometric instrument from the systematic review by Thomson, [5] and a forward citation search for the original study by Hiss and colleagues (1978). The majority of previous studies provided inadequate details of the methods of identifying opinion leaders, partly due to editorial pressures on space (Soumerai S, personal communication.). The number of opinion leaders identified varied. In the studies by Stross [10-12]], Lomas [13] and Soumerai [14], the individual with the greatest number of nominations per institution was identified as an opinion leader. In the other studies, a larger number of opinion leaders were identified (similar to the current study). These differences are probably due to different strategies for analysing the sociometric instrument. Coverage rates are rarely reported, although Lomas [13] and Soumerai [14] both report that the identified opinion leaders received the clear majority of votes within their hospital. As a result, it is difficult to assess the coverage likely to be needed if the strategy is successful. All of these factors have important implications for the utility of the method in a service setting, as it would be difficult to justify as a single strategy a method that potentially only drew on just more than half of the population and could not cover the non-responding half. We used convenience samples for this work, so it is important that the study is replicated in other settings and populations of clinicians. Indeed, it would be interesting to repeat it in the same populations in a few years to see if recent UK health reforms, with their emphasis on localities of general practitioners, have changed the situation.