Covariates
Several previous studies that have assessed the association between income inequality and self-rated health used only socio- demographic covariates (Ichida et al., 2009; Kim & Kawachi,2007; Subramanian, Delgado, Jadue, Vega, & Kawachi, 2003). Some studies also adjusted for lifestyle variables, such as smoking (Lopez, 2004; Xi, McDowell, Nair, & Spasoff, 2005, pp. 31e45) and exercise (Xi et al., 2005). Smoking is an important lifestyle variable because it is a common risk factor for poor dental status and poor self-rated health. Sex, age group (65e69 years, 70e74, 75e79,
80e84 and 85 years or older), marital status (married, separated/divorced, never married), educational attainment (years, <6, 6e9,
10e12 or 13), smoking status (never, ever, or current), individual and community level equivalent income were used as covariates in this study. Calculation of equivalent income took into account household income and number of household members. The income question had 14 categories, and the midpoints were set as house hold income in each category. We adjusted household income for household size, dividing the income by the square root of the number of people in that household. As the association between health and income has been suggested to be non-linear (Kawachi,
2000; Rodgers, 1979), the individual-level equivalent income was used as a categorical variable (ten thousand Yen, <150, 150e199,200e249, 250e299, 300e349, 350e399, 400e449, 450e499,500). The mean individual level equivalent income in each of the79 local districts determined the community-level income variable.
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