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Education, income inequality, and mortality: a multiple regression analysis

BMJ 2002; 324 doi: https://doi.org/10.1136/bmj.324.7328.23 (Published 05 January 2002) Cite this as: BMJ 2002;324:23

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INDIVIDUAL-LEVEL EDUCATION DOES NOT EXPLAIN THE ASSOCIATION OF STATE-LEVEL INCOME INEQUALITY WITH HE

INDIVIDUAL-LEVEL EDUCATION DOES NOT EXPLAIN THE ASSOCIATION OF STATE-
LEVEL INCOME INEQUALITY WITH HEALTH

Muller (2002) has shown in an ecological study that lack of high
school education accounts for the state-level association of income
inequality with mortality in the US.[1] The implicit inference is that
education at the individual-level is responsible for the income inequality
association. However, 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.
Multi-level study designs overcome this limitation by including data at
both the individual and ecological levels.[2 3]

We have previously reported results for the association of income
inequality at the state-level with self-rated health using Current
Population Survey (CPS) data.[4 5] However, we have not previously
reported the effect of controlling for education at the individual-level
among adults in the 1995 and 1997 CPS as shown in the Table below.
Controlling for education attenuated but did not completely explain the
relationship 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 (e.g. average educational
attainment, historical and political features that vary by states in the
US) or other individual-level characteristics such as lifestyle.

Table: Odds ratios of fair/poor self-rated health by quintile of 
state-level income inequality, with and without adjustment for individual 
education

Quintile of 	         Model 1	      Model 2
inequality

White males (n=59,341)				
High	             1.22 (1.00-1.50)	1.16 (0.97-1.39)
Medium-high	     1.44 (1.18-1.76)	1.38 (1.15-1.65)
Medium	             1.25 (1.02-1.54)	1.20 (0.99-1.45)
Medium-low	     0.99 (0.79-1.25)	0.98 (0.79-1.22)
Low	             1.00 		1.00 

White females (n=62,404)				
High	             1.29 (1.01-1.64)	1.19 (0.95-1.48)
Medium-high	     1.41 (1.12-1.78)	1.34 (1.08-1.65)
Medium	             1.40 (1.09-1.78)	1.33 (1.07-1.66)
Medium-low	     1.11 (0.86-1.45)	1.11 (0.87-1.41)
Low	             1.00		1.00

Model 1 = Age and income as covariates at the individual-level, and 
quintile of average income as covariate at state-level
Model 2 = Model 1 plus individual-level education

REFERENCES

1. Muller A. Education, income inequality, and mortality: a multiple
regression analysis. BMJ 2002;324:23-25.

2. Diez-Roux A. Bringing context back into epidemiology: variables and
fallacies in multilevel analysis. Am J Public Health 1998;88:216-222.

3. Blakely T, Woodward A. Ecological effects in multi-level studies. J
Epidemiol Community Health 2000;54:367-374.

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.

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.

Competing interests: Table: Odds ratios of fair/poor self-rated health by quintile of state-level income inequality, with and without adjustment for individual educationQuintile of Model 1 Model 2inequalityWhite males (n=59,341) High 1.22 (1.00-1.50) 1.16 (0.97-1.39)Medium-high 1.44 (1.18-1.76) 1.38 (1.15-1.65)Medium 1.25 (1.02-1.54) 1.20 (0.99-1.45)Medium-low 0.99 (0.79-1.25) 0.98 (0.79-1.22)Low 1.00 1.00 White females (n=62,404) High 1.29 (1.01-1.64) 1.19 (0.95-1.48)Medium-high 1.41 (1.12-1.78) 1.34 (1.08-1.65)Medium 1.40 (1.09-1.78) 1.33 (1.07-1.66)Medium-low 1.11 (0.86-1.45) 1.11 (0.87-1.41)Low 1.00 1.00Model 1 = Age and income as covariates at the individual-level, and quintile of average income as covariate at state-levelModel 2 = Model 1 plus individual-level education

13 March 2002
Tony A Blakely
Senior Research Fellow
Ichiro Kawachi
Wellington School of Medicine and Health Sciences, University of Otago