Evidence-based public health: not only whether it works, but how it can be made to work practicably at scale

Because public health must operate at scale in widely diverse, complex situations, randomized controlled trials (RCTs) have limited utility for public health. Other methodologies are needed. A key conceptual backbone is a detailed "theory of change" to apply appropriate evidence for each operational component. Synthesizing patterns of findings across multiple methodologies provides key insights. Programs operating successfully across a variety of settings can provide some of the best evidence. Challenges include judging the quality of such evidence and assisting programs to apply it. WHO and others should shift emphasis from RCTs to more relevant evidence when assessing public health issues.