Assessing the causality of the link: back to basics
In 1965, Sir Bradford Hill, an epidemiologist, proposed to consider the following criteria when assessing the causality of a relationship between two events/conditions [2]: 1) the strength of the association, as assessed by statistics (rate ratios, risk ratios and odds ratios); 2) consistency and repeatability of the association; 3) specificity of the relationship (one cause leads to one effect); 4) temporal relationship (the cause precedes the consequence); 5) presence of a dose–response relationship, i.e. greater exposure is associated with greater incidence of the effect; 6) biological plausibility; 7) coherence between epidemiological and biological/laboratory evidence; 8) experimental evidence; and 9) analogy with the effect of other, similar factors. Two other factors need to be considered: 1) the possible involvement of bias by confounding factors in the observed association(s) [3]; and 2) the specific issue of confirmation of observations by interventional approaches [4]. The need to consider the criteria of HILL [2] as a guide rather than as a definitive tool has been underlined previously, especially in the area of nutritional epidemiology [5]. Bearing these considerations in mind, what is the evidence linking diet and COPD occurrence, and how can other dietary components or non-dietary factors confound the possible link between bean consumption and COPD?