Richard Doll in the 1950s he led the research that eventually confirmed the link between smoking and lung cancer. In 1965 he set out a list of criteria that needed to be considered before concluding that an observed link between an exposure and an outcome was causal, where an exposure might comprise anything from chemicals in the environment to habits such as smoking or lack of exercise. These criteria have been subsequently much debated, and the version shown below was developed by Jeremy Howick and colleagues, separated into what they call direct, mechanistic and parallel evidence.12
