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Epidemiological studies of chronic disease: maladjustment of observed mortality rates.

Age adjustment of observed mortality and morbidity rates is not a substitute for age-specific analysis. Measures of association between potential causal factors and adjusted mortality rates are functions of the particular adjustment procedure and the choice of reference population. We exhibit here the wide variation in simple correlation statistics that occurs with eight adjustment methods and three reference populations. We then generalize these results to the multivariate situation showing an example in which there is coherent structure for the associations between predictors and mortality. This is contrasted with another example in which no such meaningful pattern exists. Studies are cited that could have been improved by greater attention to the underlying structure of age-adjusted rates. Age adjustment of total observed rates yields meaningless numbers that are useful for comparative purposes only. Total observed rates have substantive meaning but provide useful etiological clues primarily when supported by analyses of appropriate age-specific data.