MISCLASSIFICATION OF SOCIAL DISADVANTAGE BASED ON GEOGRAPHICAL AREAS - COMPARISON OF POSTCODE AND COLLECTORS DISTRICT ANALYSES

Background. Analysis of socioeconomic status (SES), defined on the bas is of geographical area, will always be subject to misclassification o f individuals. We studied the relationship between SES and selected he alth-related measures when SES was defined firstly on the basis of pos tcode and secondly on the basis of the smaller spatial area of collect or's district (CD). Method. A Census population of 1.4 million was use d to investigate the misclassification of individuals to SES group usi ng postcode as opposed to CD. A field survey of 1000 respondents and a mortality register of 1756 deaths were used to compare the relationsh ip between SES and certain outcome variables, when SES group was assig ned using postcode and CD. Misclassification probability matrices were used to try to adjust the postcode-based results to approximate CD-ba sed results. Results. The Census data showed that nearly 50% of reside nts were misclassified into SES groups by the postcode-based system co mpared with a CD-based system. In comparing the most socially disadvan taged group with the least disadvantaged group, the postcode analysis underestimated the absolute effects of SES by 58% for the increased pr evalence of smoking, by 19% for the reduced prevalence of participatio n in junior sporting clubs and by 13% for the increased mortality rate at ages 0-64 years. Adjustment of postcode-based results using miscla ssification matrices proved fruitless due to differential misclassific ation and technical difficulties. Conclusions. Misclassification of in dividuals to SES groups on the basis of postcode has caused an underes timation of the true relationship between SES and health-related measu res. A reduction of this misclassification by using smaller spatial ar eas, such as CD or census enumeration districts, will provide improved validity in estimating the true relationship.