Specificity of association in analysis of mortality and inference on causality

Specificity of association between putative risk factor and disease under study is important to inference on causality. Nevertheless many studies investigate mortality of a single disease without comparison with a control. Age-standardized proportional mortality ratios make single disease studies into case-control studies and thus demonstrate whether or not associations are disease specific. Comparison of disease-specific with all-cause mortality experiences of whole populations classified by exposure, clearly distinguishes between exposures associated with more death and with earlier/younger death, thereby overcoming an important limitation of the familiar standardized mortality ratio (SMR). Smoking is associated with more death from lung cancer (lifetime cause-specific proportions, never 1%, light 6%, moderate 8% and heavy 12%) and with earlier/younger death from ischaemic heart disease (never 35%, light 34%, moderate 32% and heavy 29%).