People with a learning disability, the experts of their own experience are increasingly involved in research. We will be discussing in this paper their centrality in the development of a psychological therapy outcome measure for people with learning disabilities. Their involvement needs to go beyond giving their views to being included in the whole research process. It is anticipated that such participatory research will help create a measurement tool that has greater meaning and validity for people with a learning disability. We are adapting clinical outcomes in routine evaluation-outcome measure (CORE-OM) Evans et al. (2000, "J Ment Health," 9, 247), the dominant psychotherapy outcome measure in routine use in the UK, for this purpose. However the existing four domains of CORE-OM (well being, problems/symptoms, functioning and risk) do not address some of the issues and feelings that impact on the lives of people with a learning disability. This we call the "missing domain" and it is the focus of this present research. Within the Collaborative Research Group (CoRG) we are seeking to uncover the essential elements of this missing domain and convert them into items for a new domain. This article examines some of the issues involved in the complex interrelationship between the process of research and the outcome of research.