Dr David Ogilvie, Programme Leader and MRC Investigator in the Population Health Interventions group, talks to Unit Head of Communications Oliver Francis. They discuss how we evaluate complex interventions and phenomena; how we put together the findings of multiple studies in a meaningful way; and what we do with all this evidence if we want to improve public health. David reflects on his career studying physical activity, health and the built environment, and what we can learn from the Covid pandemic… and Sherlock Holmes