EVALUATING EXPOSURE CUTPOINT BIAS IN EPIDEMIOLOGIC STUDIES OF ELECTRIC AND MAGNETIC-FIELDS

Epidemiologists who study the association between exposure to electric or magnetic fields and adverse health outcomes often classify their s ubjects as ''exposed'' and ''unexposed,'' and they report results base d on an odds ratio. The exposure classification rule-or dichotomy rule -is typically based on a priori assumptions or arbitrary consideration s. We show that results may vary substantially with selection of diffe rent cutpoints by which to dichotomize exposure. Further, interpretati on and comparison of studies is dependent on the choice of cutpoint. W e suggest the use of probability plots as a more informative method of data representation. To demonstrate the utility of probability plots, we re-analyze data reported by Savitz et al. [1988, Am J Epidemiol 12 8:21-38]. Using a higher exposure cutpoint than that of the original a nalysis, we obtained larger odds ratios, two of which achieved statist ical significance. More important, probability plots of these data sho wed 1) consistency of results with measures of magnetic fields in both low- and high-power-use situations, and 2) discordance with results b ased on measures of electric fields. Given these observations, we reco mmend further study, especially that focused on the most highly expose d individuals.