Editor—Muller has shown in an ecological study that lack of high school education accounts for the association of income inequality with mortality at state level in the United States.1 The implicit inference is that education at the level of the individual is responsible for the association with inequality of income.
But ecological studies are weak study designs to assess the independent associations of ecological (income inequality) and individual level (education) variables with an individual level outcome (health status). In particular, aggregate data are prone to problems of collinearity, rendering it difficult to tease apart independent effects. Multilevel study designs overcome this limitation by including data at both individual and ecological levels.2,3
We have previously reported results for the association of income inequality at state level with self rated health using data from the current population survey data.4,5 We have not previously reported the effect of controlling for education at individual level among adults in the 1995 and 1997 survey as shown in the table. Controlling for education attenuated but did not completely explain the relation between levels of state income inequality and self rated health. Our results do not support the contention that education at the individual level fully confounds or mediates the association of income inequality with health. The remaining portion of the income inequality association may be due to contextual effects (for example, average educational attainment, or historical and political features that vary by state in the United States) or other individual level characteristics such as lifestyle.
Table.
Odds ratios (95% confidence intervals) fair to poor self rated health by fifth of state income inequality with and without adjustment for income and education
| Fifth of inequality | Adjusted for age and income | Adjusted for age, income, and education |
|---|---|---|
| White men (n=59 341 | ||
| High | 1.22 (1.00 to 1.50) | 1.16 (0.97 to 1.39) |
| Medium high | 1.44 (1.18 to 1.76) | 1.38 (1.15 to 1.65) |
| Medium | 1.25 (1.02 to 1.54) | 1.20 (0.99 to 1.45) |
| Medium low | 0.99 (0.79 to 1.25) | 0.98 (0.79 to 1.22) |
| Low | 1.00 | 1.00 |
| White women (n=62 404) | ||
| High | 1.29 (1.01 to 1.64) | 1.19 (0.95 to 1.48) |
| Medium high | 1.41 (1.12 to 1.78) | 1.34 (1.08 to 1.65) |
| Medium | 1.40 (1.09 to 1.78) | 1.33 (1.07 to 1.66) |
| Medium low | 1.11 (0.86 to 1.45) | 1.11 (0.87 to 1.41) |
| Low | 1.00 | 1.00 |
References
- 1.Muller A. Education, income inequality, and mortality: a multiple regression analysis. BMJ. 2002;324:23–25. doi: 10.1136/bmj.324.7328.23. . (5 January.) [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Diez-Roux A. Bringing context back into epidemiology: variables and fallacies in multilevel analysis. Am J Public Health. 1998;88:216–222. doi: 10.2105/ajph.88.2.216. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Blakely T, Woodward A. Ecological effects in multi-level studies. J Epidemiol Community Health. 2000;54:367–374. doi: 10.1136/jech.54.5.367. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Blakely T, Kennedy B, Kawachi I. Socio-economic inequality in voting participation and self-rated health. Am J Public Health. 2001;91:99–104. doi: 10.2105/ajph.91.1.99. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.Blakely T, Kennedy B, Glass R, Kawachi I. What is the lag time between income inequality and heath status? J Epidemiol Community Health. 2000;54:318–319. doi: 10.1136/jech.54.4.318. [DOI] [PMC free article] [PubMed] [Google Scholar]