Do confounding or selection factors of residential wiring codes and magnetic fields distort findings of electromagnetic fields studies?

In contrast with several previous studies, our recent large case-control st udy found little association between childhood acute lymphoblastic leukemia (ALL) and electric-power-line wire codes. Here we examine internal evidenc e from our study to assess the possibility that selection bias and/or confo unding may have affected the findings. We compared the relation between chi ldhood ALL and wire codes and direct measurements of magnetic fields in sub jects 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 hig h current configurations increased by 23% when 107 "partial participants" w ere 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 poss ible selection bias. We also examined the relation between a large number o f potential confounding variables and both proxy and direct measurements of magnetic fields. Univariate adjustment for individual variables changed th e odds ratio for ALL by less than 8%, while simultaneous adjustment for sev eral factors reduced the estimate by a maximum of 15%. We conclude that whi le confounding alone is unlikely to be an important source of bias in our o wn and previous studies of magnetic fields, selection bias may be more of a concern, particularly in light of the generally low response rates among c ontrols in case-control studies.