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

Abstract

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 a mainstay of epidemiological textbooks and data interpretation. Their checklist nature suited the study of simple direct causation by disciplines characterized by classic scientific and mathematical training. Most diseases have a multifactorial pathogenesis but the conceptualization of their causation varies by discipline. While it is scientifically satisfying to elucidate the many component causes of an illness in public health research the more important emphasis is on the discovery of necessary or sufficient causes that are amenable to intervention. Even so over the four decades since Bradford Hill’s paper appeared the range of multivariate multistage and multi-level research questions tackled by epidemiologists has evolved as have their statistical methods and their engagement in wider-ranging interdisciplinary research. Within that context it is often not appropriate to seek the discrete cause or causes of a disease but rather to identify a complex of interrelated and often interacting factors that influence the risk of disease. This complicates the assessment of causality. (excerpt)