American Journal of Epidemiology Advance Access published online on May 15, 2008
American Journal of Epidemiology, doi:10.1093/aje/kwn086
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Practice of Epidemiology |
Assessment of Selection Bias in the Canadian Case-Control Study of Residential Magnetic Field Exposure and Childhood Leukemia
1 Electric Power Research Institute, Palo Alto, CA
2 British Columbia Cancer Agency, Vancouver, British Columbia, Canada
3 PW International, Inc., Burnaby, British Columbia, Canada
Correspondence to Dr. Gabor Mezei, 3420 Hillview Avenue, Palo Alto, CA 94304 (e-mail: gmezei{at}epri.com).
Received for publication November 12, 2007. Accepted for publication March 11, 2008.
The authors evaluated the role of selection bias in the 1999 Canadian case-control study of residential magnetic field exposure and childhood leukemia. They included cases, participating controls, and first-choice nonparticipating controls in their analyses. Exposure was assessed by wire coding, a classification system based on the distribution line characteristics near homes. Although an imperfect measure of magnetic field exposure, wire coding is the only method applicable to nonparticipating subjects. First-choice nonparticipant controls tended to be of lower socioeconomic status than their replacements (non-first-choice participant controls), and lower socioeconomic status was related to higher wire code categories. The odds ratios for developing childhood leukemia in the highest exposure category were 1.6 (95% confidence interval: 1.0, 2.6) when the actual participating controls were used and 1.3 (95% confidence interval: 0.8, 2.1) when the first-choice ideal controls were used, regardless of their participation. Overall, the authors conclude that, although there is some evidence for control selection or participation bias in the Canadian study, it is unlikely to explain entirely the observed association between magnetic field exposure and childhood leukemia. Inherent problems in exposure assessment for nonparticipating subjects, however, limit the interpretations of these results, and the role of selection bias cannot entirely be dismissed on the basis of these results alone.
case-control studies; child; electromagnetic fields; epidemiologic methods; leukemia; selection bias
Abbreviations: CI, confidence interval; IPPE, income per person equivalent; OR, odds ratio
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