What is the impact of reducing air pollution on mortality? And how does this compare to reducing smoking? Or the impact of salt reduction for preventing cardiovascular deaths? The answer depends on the perspective: effects may not be relevant from the individual perspective, but could be relevant from the population perspective when there is an important number of preventable deaths. These videos, originally part of the Cochrane Learning Live webinar series, explain how presentation of effect sizes from the individual or population perspective can influence judgments about the importance of these effects and prevention strategies. Dr Jos Verbeek provides guidance for systematic review authors on how to consider the individual and the population perspective. Part 1: Presentation Part 2: Questions and answers