Misclassification of social disadvantage based on geographical areas: comparison of postcode and collector's district...

Analysis of socioeconomic status (SES), defined on the basis of geographical area, will always be subject to misclassification of individuals. We studied the relationship between SES and selected health-related measures when SES was defined firstly on the basis of postcode and secondly on the basis of the smaller spatial area of collector's district (CD).

A Census population of 1.4 million was used to investigate the misclassification of individuals to SES group using postcode as opposed to CD. A field survey of 1000 respondents and a mortality register of 1756 deaths were used to compare the relationship between SES and certain outcome variables, when SES group was assigned using postcode and CD. Misclassification probability matrices were used to try to adjust the postcode-based results to approximate CD-based results.

The Census data showed that nearly 50% of residents were misclassified into SES groups by the postcode-based system compared with a CD-based system. In comparing the most socially disadvantaged group with the least disadvantaged group, the postcode analysis underestimated the absolute effects of SES by 58% for the increased prevalence of smoking, by 19% for the reduced prevalence of participation in junior sporting clubs and by 13% for the increased mortality rate at ages 0-64 years. Adjustment of postcode-based results using misclassification matrices proved fruitless due to differential misclassification and technical difficulties.

Misclassification of individuals to SES groups on the basis of postcode has caused an underestimation of the true relationship between SES and health-related measures. A reduction of this misclassification by using smaller spatial areas, such as CD or census enumeration districts, will provide improved validity in estimating the true relationship.