Academic journal article Canadian Journal of Public Health

Ecologic Proxies for Household Income: How Well Do They Work for the Analysis of Health and Health Care Utilization?

Academic journal article Canadian Journal of Public Health

Ecologic Proxies for Household Income: How Well Do They Work for the Analysis of Health and Health Care Utilization?

Article excerpt


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.

Personal or household incomes are important correlates of health status and the utilization of health services in Canada,1 the United States,2 and Europe.3 Information on personal measures of income and socio-economic status (SES) is occasionally available to researchers, particularly when analyzing national surveys such as the National Population Health Survey (NPHS). In many cases, however, personal information is not available, and researchers are obliged to infer personal SES from ecologic measures obtained, for example, from census data.4

Canadian researchers make use of administrative data to study questions pertinent to health care and public health policy.5,6 There has been worry about the possibility of misclassification of SES in the use of these data. Demissie7 found substantial discrepancy between area-based SES measures and SES assessed at the individual level in Montreal. Sin8 studied the validity of using postal codes to identify groups of people with low socio-economic status. he reported that the use of Forward Sortation Area (FSA) postal codes as the only marker to identify people with low SES may result in substantial misclassification of personal poverty. Glazier9 reported that, although address inaccuracy was found in Ontario's health care registry, serious socio-economic misclassification occurred at a low rate and did not introduce significant bias in the calculation of hospitalization rates by socio-economic group. Southern et al.10 assessed the agreement between FSA and Enumeration Area (EA) derived income levels in Alberta. They found that the variability in EA-derived income quintiles was large for any given FSA-derived income quintile and recommended that EA-derived measures should be used when individual data are not available.

Measurement theory predicts that a group level measure will contain more measurement error than the direct household measure. Accordingly, the relationship between the group level measure and a health outcome will be attenuated in comparison to the true relationship relative to the direct measure of socio-economic status.11 Mustard and colleagues12 examined the validity of using ecologie measures of SES as proxies for individual-level measures by using a 5% sample of households in Manitoba and linking the records of the Manitoba Health Services Insurance Plan and the Census of 1986. They concluded that the hypothesis that risk estimates derived from ecologie income measures will be attenuated relative to estimates obtained from household income was not supported. …

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