Causation, bias and confounding: a hitchhiker's guide to the epidemiological galaxy

In Part 1 of this series the following principles of causation were considered: time order, specification of the study base, and specificity (bias due to random misclassification and bias due to systematic misclassification). 1 Part 2 will consider: 2a: Confounding 2b: Effect modification 2c: Strength of association. Part 3 concludes the consideration of causal principles and will discuss statistical stability, dose-and durationresponse effects, internal consistency, external consistency, analogy and biological plausibility. 2 2a: Confounding Confounding exists when a risk factor other than the exposure under study is associated, independently, both with the exposure and with the outcome. Confidence in causality is increased if an association can be judged to be reasonably unconfounded. When present, confounding is the same in a follow-up or case-control study carried out in the same study base. To the degree that confounding can be precisely measured, its effect can be removed or minimised.