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

Study objectives-To study geographical differences in diastolic blood press ure and the influence of the social environment (census percentage of peopl e 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 lev el, and by single level ecological regression using areas as the unit of an alysis. 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 (bet a =2.966, SE=1.250) were independently associated with blood pressure, alth ough in the "random effects" multilevel analysis almost none of the total v ariability in blood pressure across persons was attributable to areas (intr aclass correlation=0.3%). The ecological analysis also found an association between the area educational variable and mean diastolic blood pressure (b eta =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 indiv idual blood pressure. However, these slight differences were enough to dete ct an effect of the social environment on blood pressure. The ecological st udy overestimated the associations found in the "fixed" effects multilevel analysis, and neither distinguished individual from area levels nor provide d information on the intraclass correlation. Ecological analyses are inadeq uate to evaluate geographical differences in health.