Bias in epidemiological studies of EMFs
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 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 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, eg, 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.
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