EFFECTS OF THE CHOICE OF AGE-ADJUSTMENT METHOD ON MAPS OF DEATH RATES

Maps of morbidity or mortality rates, whether considered individually or as a layer in a geographic information system application, invite m ultiple comparisons of area rates. However, comparisons of rates acros s different populations require standardization of the age-specific ra tes to account for differences in population age structures. The indir ect standardization method, or equivalently the standardized mortality ratio (SMR), has been recommended for small areas where age-specific rates can be quite variable. Although theoretically equivalent to dire ctly adjusted rates under the assumption of independent age and area e ffects, indirect summary measures are not comparable across areas when this assumption is violated. We tested the validity of this assumptio n for the 10 most common causes of death in the United States during 1 980-84 and examined the geographic clustering apparent when categorize d death rates, adjusted by different methods, are presented as themati c maps. Although overall agreement between the methods was good (rank correlation coefficient > 82 per cent for each cause), when the adjust ed rates were classified into quintiles 18 per cent of the states fell into different categories depending on the method of adjustment. Usin g an internal standard for the indirect method reduced this discrepanc y to 4.9 per cent. However, both traditional chi-square tests and a ge neralized logistic spline model identified significant interactions be tween age and area for each cause of death, a violation of the assumpt ion required for equivalence of the methods. Potential variation in ge ographic inferences is illustrated by maps of direct and indirect rate s and an empirical Bayes posterior mean, which is a function of these traditionally adjusted rates. Based on these results, we recommend the direct age-adjustment method for rate maps.