Exposure assessment in investigations of waterborne illness: a quantitative estimate of measurement error.
BACKGROUND
Exposure assessment is typically the greatest weakness of epidemiologic studies of disinfection by-products (DBPs) in drinking water, which largely stems from the difficulty in obtaining accurate data on individual-level water consumption patterns and activity. Thus, surrogate measures for such waterborne exposures are commonly used. Little attention however, has been directed towards formal validation of these measures.


METHODS
We conducted a study in the City of Hamilton, Ontario (Canada) in 2001-2002, to assess the accuracy of two surrogate measures of home water source: (a) urban/rural status as assigned using residential postal codes, and (b) mapping of residential postal codes to municipal water systems within a Geographic Information System (GIS). We then assessed the accuracy of a commonly-used surrogate measure of an individual's actual drinking water source, namely, their home water source.


RESULTS
The surrogates for home water source provided good classification of residents served by municipal water systems (approximately 98% predictive value), but did not perform well in classifying those served by private water systems (average: 63.5% predictive value). More importantly, we found that home water source was a poor surrogate measure of the individuals' actual drinking water source(s), being associated with high misclassification errors.


CONCLUSIONS
This study demonstrated substantial misclassification errors associated with a surrogate measure commonly used in studies of drinking water disinfection byproducts. Further, the limited accuracy of two surrogate measures of an individual's home water source heeds caution in their use in exposure classification methodology. While these surrogates are inexpensive and convenient, they should not be substituted for direct collection of accurate data pertaining to the subjects' waterborne disease exposure. In instances where such surrogates must be used, estimation of the misclassification and its subsequent effects are recommended for the interpretation and communication of results. Our results also lend support for further investigation into the quantification of the exposure misclassification associated with these surrogate measures, which would provide useful estimates for consideration in interpretation of waterborne disease studies.
Jones
Andria Q
AQ
Division of Community Health and Humanities, Faculty of Medicine, Memorial University of Newfoundland, The Health Sciences Centre, St. John's, Newfoundland, A1B 3V6, Canada.
Dewey
Catherine E
CE
Department of Population Medicine, University of Guelph, 50 Stone Road West, Guelph, Ontario, N1G 2W1, Canada.
Doré
Kathryn
K
Department of Population Medicine, University of Guelph, 50 Stone Road West, Guelph, Ontario, N1G 2W1, Canada.|Foodborne, Waterborne and Zoonotic Infections Division, Public Health Agency of Canada (160 Research Lane, Suite 206 Guelph, Ontario, N1G 5B2, Canada.
Majowicz
Shannon E
SE
Department of Population Medicine, University of Guelph, 50 Stone Road West, Guelph, Ontario, N1G 2W1, Canada.|Foodborne, Waterborne and Zoonotic Infections Division, Public Health Agency of Canada (160 Research Lane, Suite 206 Guelph, Ontario, N1G 5B2, Canada.
McEwen
Scott A
SA
Department of Population Medicine, University of Guelph, 50 Stone Road West, Guelph, Ontario, N1G 2W1, Canada.
Waltner-Toews
David
D
Department of Population Medicine, University of Guelph, 50 Stone Road West, Guelph, Ontario, N1G 2W1, Canada.