Ecologic proxies for household income: how well do they work for the analysis of health and health care utilization?

BACKGROUND Researchers often use census-derived measures of socioeconomic status (SES) when personal information is not available. Theory predicts that the resulting misclassification will blunt associations between outcomes and SES and that control for confounding by SES will be less effective. The purpose of this paper was to examine the magnitude of this problem using data from the National Population Health Survey (NPHS). METHODS Subjects were 4,037 respondents to the NPHS who were linked to the Ontario Health Insurance Plan. An ecologic measure of income was obtained by linkage of subjects' postal codes to the Census. RESULTS The relationships between the ecologic-level measure and health outcomes or health services utilization were attenuated in comparison to the relationships relative to the direct measure of household income. The ecologic measure also produced poorer control for confounding by income in the analysis of other health relationships. CONCLUSIONS Many interesting public health and health services questions can be addressed only with the use of ecologic level socioeconomic information. While most of the results were qualitatively similar when the direct and ecologic measures were compared, researchers and users of research findings should be aware that attenuated or potentially misleading findings may result from the use of these methods.