BMJ  2005;331:635 (17 September), doi:10.1136/bmj.331.7517.635

Letter

Childhood cancer and power lines

Results depend on chosen control group

The first 150 words of the full text of this article appear below.

Editor—Given the large size of the study by Draper et al,¹ the risk estimates should be stable. Furthermore, because contact with the subject was not necessary, selection bias due to the differential participation among cases and controls as in previous studies has been avoided.²

We were therefore surprised by the dependence of the results on the chosen control group noted by the authors (who used the central nervous system and other cancer controls for leukaemia cases in one of the comparisons). To explore this further, we combined all controls into one group and used it for comparison. We thought this was justified on the basis of theoretical and empirical grounds: exposure at birth among controls chosen for leukaemia, brain tumours, and other cancers should not depend on the cancer subtype; crude odds ratios calculated by us did not differ (beyond the first decimal) from the matched results presented by the . . . [Full text of this article]

UCLA School of Public Health, Department of Epidemiology, 73-284 CHS, 650 Charles E Young Drive South, Los Angeles, CA 90095-1772, USA kheifets@ucla.edu

Institute of Environmental Medicine, Karolinska Institutet, Box 210, S-17177 Sweden

Department of Biostatistics and Epidemiology, Institute of Cancer Epidemiology, DK-2100 Copenhagen, Denmark

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