Diastolic blood pressure and area of residence:: multilevel versus ecological analysis of social inequity

Study objectives-To study geographical differences in diastolic blood pressure and the influence of the social environment (census percentage of people with low educational achievement) on individual diastolic blood pressure level, after controlling for individual age and educational achievement. To compare the results of multilevel and ecological analyses. Design - Cross sectional analysis performed by multilevel linear regression modelling, with women at the first level and urban areas at the second level, and by single level ecological regression using areas as the unit of analysis. Setting - Malmo, Sweden (population 250000). Participants - 15 569 women aged 45 to 73, residing in 17 urban areas, who took part in the Malmo Diet and Cancer Study (1991-1996). Main results - In the "fixed effects" multilevel analysis, low educational achievement at both individual (beta =1.093, SE=0.167) and area levels (beta =2.966, SE=1.250) were independently associated with blood pressure, although in the "random effects" multilevel analysis almost none of the total variability in blood pressure across persons was attributable to areas (intraclass correlation=0.3%). The ecological analysis also found an association between the area educational variable and mean diastolic blood pressure (beta =4.058, SE=1.345). Conclusions-The small intraclass correlation found indicated very marginal geographical differences and almost no influence of the urban area on individual blood pressure. However, these slight differences were enough to detect an effect of the social environment on blood pressure. The ecological study overestimated the associations found in the "fixed" effects multilevel analysis, and neither distinguished individual from area levels nor provided information on the intraclass correlation. Ecological analyses are inadequate to evaluate geographical differences in health.