IJE Advance Access originally published online on November 22, 2005
International Journal of Epidemiology 2006 35(2):397-406; doi:10.1093/ije/dyi245
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Methodology |
Selection bias and its implications for case–control studies: a case study of magnetic field exposure and childhood leukaemia
1 Environment Department, Electric Power Research Institute, Palo Alto, CA, USA.
2 Department of Epidemiology, UCLA School of Public Health, Los Angeles, CA, USA.
* Corresponding author. Electric Power Research Institute, 3412 Hillview Avenue, Palo Alto, CA 94303, USA. E-mail: gmezei{at}epri.com
Based on the epidemiological association between residential exposure to extremely low frequency-magnetic fields (ELF-MF) and childhood leukaemia, the International Agency for Research on Cancer classified ELF-MF as a possible human carcinogen. Since clear supportive laboratory evidence is lacking and biophysical plausibility of carcinogenicity of MFs is questioned, a causal relationship between childhood leukaemia and magnetic field exposure is not established. Among the alternative explanations, selection bias in epidemiological studies of MFs seems to be the most plausible hypothesis. In reviewing the epidemiological literature on ELF-MF exposure and childhood leukaemia, we found evidence both for and against the existence of selection bias. To evaluate the potential for selection bias, we examined the relationship of socioeconomic status to subject participation and exposure to MFs. We find that, often, reporting of selection processes in itself is biased and incomplete, making the interpretation and evaluation of a potential for bias difficult. However, if present, such a bias would have wide implications for case–control studies in general. We call for better reporting and for evaluation of the potential for selection bias in all case–control studies, as well as, for the development of novel methods in control selection and recruitment.
Keywords Epidemiological methods, selection bias, childhood leukaemia, extremely low frequency-magnetic fields
Accepted 13 October 2005
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