Large scale health intervention covers a wide range of circumstances, including the use of a new drug or therapeutic procedure. In this case we have developed over the past 50 years a widely accepted understanding of the nature of the evidence that would lead to a consensus about an intervention’s use, while not underestimating the challenge of acquiring it. Other interventions are much more complex; they are about the behaviour of people and systems, and it does no service to the public to apply only the yardsticks we have developed for narrower biomedical interventions. Although we should be equally rigorous in our evaluation, we need to learn from other scientific sectors to broaden our understanding of evidence.