On the importance of age-adjustment methods 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.


METHODS
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's (2000) Social Capital Index was downloaded from http://www.bowlingalone.com; the Social Mistrust Index was calculated from responses to the General Social Survey, following the method described in Kawachi et al. (1997). All other covariates are obtained from the U.S. Census Bureau.


METHODS
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 from regressions of crude mortality on age variables.


RESULTS
Different age-adjustment methods can cause a change in the sign or statistical significance of the association between mortality and various socioeconomic factors. When age variables are included as regressors, we find no significant 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.
Milyo
Jeffrey
J
Harris Graduate School of Public Policy Studies, University of Chicago, IL 60637, USA.
Mellor
Jennifer M
JM
D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000367:Age Factors; D000368:Aged; D015982:Bias; D004463:Ecology; D006801:Humans; D008875:Middle Aged; D009026:Mortality / Q000639:trends*; D011203:Poverty; D020379:Risk Adjustment; D012959:Socioeconomic Factors; D012962:Sociology, Medical; D014481:United States / Q000453:epidemiology