Trial designs and exploration of the placebo response

The 'placebo response' is a concept derived from the use of dummy (sham) interventions as 'controls' for active interventions within the context of a randomised controlled trial (RCT). Clearly if there is nothing in the sham treatment it can have no effect, so the response must be dictated by other contextual or incidental factors. However, the assumptions and processes that underlie the classical RCT make it difficult to explore these incidental factors, leaving us with the paradox that while the RCT defines placebos, we cannot easily explore placebo responses within RCTs. Furthermore, complexity makes the 'simple' RCT an inadequate approach to assess interventions in chronic diseases. A variety of alternative trial designs (such as stepped wedge designs, pre-randomisation and cluster randomisation) are discussed. Different approaches, including nested qualitative research and realist research approaches, are recommended as ways forward for the investigation of the placebo response.