Public involvement in research has evolved over the last two decades in a culture dominated by the principles of evidence-based medicine. It is therefore unsurprising that some researchers have applied the same thinking to involvement, particularly to involvement in research projects. This may explain why they tend to conceptualise involvement as an intervention, seek to evaluate its impact in the same way that treatments are tested, highlight the need for an evidence-base for involvement, and use the language of research to describe its practice and report its outcomes. In this article we explore why this thinking may be unhelpful. We suggest an alternative approach that conceptualises involvement as 'conversations that support two-way learning'. With this framing, there is no 'method' for involvement, but a wide range of approaches that need to be tailored to the context and the needs of the individuals involved. The quality of the interaction between researchers and the public becomes more important than the process. All parties need to be better prepared to offer and receive constructive criticism and to engage in constructive conflict that leads to the best ideas and decisions. The immediate outcomes of involvement in terms of what researchers learn are subjective (specific to the researcher) and unpredictable (because researchers don't know what they don't know at the start). This makes it challenging to quantify such outcomes, and to carry out comparisons of different approaches. On this basis, we believe obtaining 'robust evidence' of the outcomes of involvement in ways that are consistent with the values of evidence-based medicine, may not be possible or appropriate. We argue that researchers' subjective accounts of what they learnt through involvement represent an equally valid way of knowing whether involvement has made a difference. Different approaches to evaluating and reporting involvement need to be adopted, which describe the details of what was said and learnt by whom (short term outcomes), what changes were made as a result (medium term outcomes), and the long-term, wider impacts on the research culture and agenda. Sharing researchers' personal accounts may support wider learning about how involvement works, for whom and when.
Keywords: Evaluation; Good practice; Impact; Patient and public involvement; Public involvement.