Description of a new computer wire coding method and its application to evaluate potential control selection bias in the Savitz et al. childhood cancer study

We developed a new computer wire coding method and then applied it to inves tigate the suggestion that control selection bias might explain the observe d association between wire codes and childhood cancer made in the study con ducted by Savitz et at. in the Denver area. The computer wire coding method used a geographic information system approach with data on the local distr ibution electric system and from tax assessor records. Individual residence s were represented as a circle scaled to the ground floor area of the resid ence and centered on the lot centroid. The wire code of thr residence was d etermined from the distance between the circle and the relevant power line, and from the current carrying capacity of that line. Using this method, wi re codes were generated for 238 290 residences built before 1986, the time of the Savitz et al. study, in the Denver metropolitan area. We then attemp ted to reconstruct the 1985 population of hypothetically eligible control c hildren in the Denver metropolitan area by using 1980 census data. Since da ta were not available to locate the children in each residence within a cen sus block, uniform, Poisson, and negative binomial distributions were used to randomly assign children to residences. To evaluate the Likelihood of th e wire code distribution of the controls selected by Savitz et al., 100 ran dom trials were conducted for each distribution, matching two controls to e ach case. The odds ratios between childhood cancer and very high current co nfiguration (VHCC) wire codes were reduced when the assigned controls were used, suggesting control selection bias may have been present. However, con trol selection bias is unlikely to account for all the reported association between childhood cancer and wire codes in the Savitz et al, study. (C) 20 00 Wiley-Liss, Inc.