Epidemiological studies have found a statistical association between magnetic fields and childhood leukaemia. This might mean that magnetic fields cause leukaemia but there are other explanations as well. One of these is bias in the studies. The commonest type of bias is when some people choose to participate in the studies but others decline. If more people with higher socioeconomic status – who tend to have lower fields – agree to participate, and this happens more in the “control” children than in the cases, this can create a bias.
Scientists agree there is scope for bias in some of the studies of EMFs but no-one knows if it explains the results or not. A recent paper noted below by Mezei & Kheifets surveyed the arguments either way and we summarise them here.
| Why bias might explain the results | Why bias might not explain the results |
Participation rates in many of the studies were quite low and there is evidence that this was sometimes linked to socioeconomic status. A study by Hatch et al (see bottom pf page) showed that including some of the people who only participated partially in the original study reduced the association found in a reanalysis (from a relative risk of 1.9 to 1.6). The implication is that if we could adjust completely for participation, the association would have become even smaller. Bias doesn’t have to explain all the result – some could be due to chance as well . | The different participation rates between different groups would actually have to be quite large to explain the associations found in the studies. Adjusting for socioeconomic status in many of the studies made little difference to the result. There is no clear link between the participation rate of a study and the result it found. Some of the studies (those in the Nordic countries which used population registries) in the pooled analysis were free from bias and they found similar associations to the studies which do have bias. We would expect bias to apply to studies of other childhood cancers as well, but it seems to be mainly childhood leukaemia that is associated with magnetic fields, not, e.g., brain cancer. |
Epidemiology. 2000 Mar;11(2):189-98. Do confounding or selection factors of residential wiring codes and magnetic fields distort findings of electromagnetic fields studies? Hatch EE, Kleinerman RA, Linet MS, Tarone RE, Kaune WT, Auvinen A, Baris D, Robison LL, Wacholder S. Radiation Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, Rockville, MD 20892, USA. In contrast with several previous studies, our recent large case-control study found little association between childhood acute lymphoblastic leukemia (ALL) and electric-power-line wire codes. Here we examine internal evidence from our study to assess the possibility that selection bias and/or confounding may have affected the findings. We compared the relation between childhood ALL and wire codes and direct measurements of magnetic fields in subjects who participated in all phases of the study with the relation in all subjects, including those who declined to allow access inside the home. We found that the odds ratio for ALL among those living in homes with very high current configurations increased by 23% when 107 "partial participants" were excluded. We found similar, but slightly smaller, increases in the odds ratios when we performed the same comparisons using direct measurements of magnetic fields, excluding subjects who allowed only a measurement outside the front door. "Partial participants" tended to be characterized by lower socioeconomic status than subjects who participated fully, suggesting possible selection bias. We also examined the relation between a large number of potential confounding variables and both proxy and direct measurements of magnetic fields. Univariate adjustment for individual variables changed the odds ratio for ALL by less than 8%, while simultaneous adjustment for several factors reduced the estimate by a maximum of 15%. We conclude that while confounding alone is unlikely to be an important source of bias in our own and previous studies of magnetic fields, selection bias may be more of a concern, particularly in light of the generally low response rates among controls in case-control studies. |
Int J Epidemiol. 2005 Nov 22; [Epub ahead of print] Selection bias and its implications for case-control studies: a case study of magnetic field exposure and childhood leukaemia. Mezei G, Kheifets L. Environment Department, Electric Power Research Institute, Palo Alto, CA, USA. 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. |