On the Importance of Full versus Partial Age-Adjustment in Ecological Studies of Social Determinants of Mortality

Objective: To illustrate the potential sensitivity of ecological associations between mortality and certain socioeconomic factors to different methods of age-adjustment. Data Sources: Secondary analysis employing state-level data from several publicly available sources. Crude and age-adjusted mortality rates for 1990 are obtained from the U.S. Centers for Disease Control. The Gini coefficient for family income and percent of persons below the federal poverty line are from the U.S. Bureau of Labor Statistics. Putnam (2000)’s Social Capital index is downloaded from www.bowlingalone.com; the Social Mistrust index is calculated from responses to the General Social Survey, according to the method described in Kawachi et al. (1998). All other co-variates are obtained from the U.S. Census Bureau. Study Design: We use least squares regression to estimate the effect of several state-level socioeconomic factors on mortality rates. We examine whether these statistical associations are sensitive to the use of alternative methods of accounting for the different age composition of state populations. Following several previous studies, we present results for the case when only mortality rates are age-adjusted. We contrast these results with those obtained when age variables are included as controls in the regression analysis. Principle Findings: Partial age-adjustment is shown to yield a strong and significant association between mortality and each socioeconomic factor. Full age-adjustment produces no such association between mortality and either income inequality, minority racial concentration or social capital. Conclusions: Ecological associations between certain socioeconomic factors and mortality may be extremely sensitive to different age-adjustment methods.