Association or causation: evaluating links between "environment and disease".

Epidemiological studies typically examine associations between an exposure variable and a health outcome. In assessing the causal nature of an observed association the “Bradford Hill criteria” have long provided a background framework — in the words of one of Bradford Hill’s closest colleagues an “aid to thought”. First published exactly 40 years ago these criteria also provided biomedical relevance to epidemiological research and quickly became...