Layers of complexity in interpreting evidence on effectiveness.

There has been growing attention to using evidence on effectiveness to guide public health and health improvement policies, strategies, programmes and actions 'on the ground'. However, there has been insufficient recognition of the complexity attached to interpreting such evidence and translating it into action. In this paper, a randomized controlled trial is used as an illustrative case study to uncover layers of that complexity. It is suggested that these layers should be taken into account in designing, executing, analysing and reporting primary evaluative studies and reviews; formulating recommendations for action; and developing more fully fit-for-purpose approaches to evidence-informed public health and health improvement.