Many current evidence synthesis methods and processes have potential application to reviews where intervention complexity is considered important. There is however a critical lack of application of existing methods to better understands and synthesise/interpret evidence on complexity. There are considerable gaps in current methods and knowledge that require further methodological development and testing. The Cochrane Collaboration undertakes systematic reviews of complex interventions and interventions with varying degrees of complexity. The Collaboration is currently developing guidance in the form of a handbook chapter led byMark Petticrew and Laurie Anderson. Better understanding of these methodological and process issues is therefore of high importance. In addition to developing guidance for authors, the Methods Innovation Fund (MIF) has supported a number of projects to develop/apply appropriate synthesis methods and guidance for Cochrane authors. In January 2012, a meeting with methodologists was held in Montebello, Canada (partly funded by MIF), and a series of papers from this meeting will be published in parallel with the Quebec Colloquium. This proposed special session on complex interventions serves as an update to this work and an opportunity to disseminate key developments and future plans. We will set the scene, discussing why Cochrane is concerned with intervention complexity and complex interventions. We will give a brief update from the Montebello meeting and a series of papers published in JCE, and the Methodological Investigation of Cochrane Complex Interventions (MICCI) project. Mark Petticrewwill speak to the current state of the art and science of synthesis methods to understand intervention complexity and synthesise evidence from complex interventions. This will be followed with a presentation on a new tool for unpicking intervention complexity using the iCAT SR tool (Cochrane MIF project). Lastly, we will share case studies of recently published Cochrane reviews to show how complexity was identified and addressed and current gaps in methods.

Target audience and level of expertise: Any person with an interest in intervention complexity and complex interventions.