Background: Within medicine there is an agreed hierarchy of levels of research evidence with randomized controlled trials at the top and expert opinion at the bottom. When randomized controlled trials are the final stage in testing a treatment for which there is a sound theoretical basis both the 'Does it work?' and 'How does it work?' questions may be answered. However, when the theoretical basis is underdeveloped or contentious, then randomized controlled trials will provide only partial answers and other methods or methodologies may be needed to understand more about the mechanisms involved in the treatments. Aims: This paper aims to contribute to the debate about the nature of the evidence that is required in order to feel confident that people with communication difficulties will benefit from the treatments offered. The aim is not to discuss the relative merits of one methodology over another, but rather, by referring to one particular treatment, to show how knowledge gained from a range of research studies, using different methodologies, can be synthesized to increase one's understanding. Methods & Procedures: The initial discussion focuses on two frameworks that have been developed specifically for the evaluation of healthcare interventions. This is followed by discussion of the research evidence for the Lidcombe Program of early stuttering intervention to illustrate some issues relevant to treatment development and evaluation in the field of communication difficulty. The evidence from randomized controlled trails is related to clinicians' needs. Finally, the potential value of an iterative approach is illustrated with reference to verbal contingencies, which are an essential component of the Lidcombe Program. Main Contribution: This paper considers different sorts of evidence and their contribution to one's knowledge and practice. The content provides a practical example of how knowledge and understanding of a treatment approach can develop through a synthesis of knowledge from a programme of systematic research, from research outside the field of communication difficulty, and from the process of making sense of the experiences of both clinicians and clients who use the approach. Conclusions & Implications: The questions 'Does it work?' and 'How does it work?' are important and require different methods of investigation. Particular treatments may be viewed as better suited to one research methodology over another, but limiting research to one fails to provide the information that practitioners need in order to justify what they do and effectively to solve problems encountered in the application of particular approaches.
