bias. It seems preferable to tackle those questions directly by asking whether a given association suffers from distortion due to the study biases.

Third, epidemiologic results should be first evaluated on terms inherent within the discipline, referring to qualities of study design, execution, and analysis. After that, insights and knowledge derived from other scientific disciplines relevant to the association in question can most effectively contribute to judgments about causality.

CANCER EPIDEMIOLOGY-RESIDENTIAL EXPOSURES

Summary of Evidence

The studies that have provided empirical evidence relating residential magnetic-field exposure to cancer are summarized in a series of tables in Appendix A (Tables A5-1, A5-2, and A5-3) that address the study methods. Later in this chapter the methodologic issues are critically evaluated, but this section is intended to provide a summary of the study structure (Table A5-1), of the methods used in control selection in case-control studies (Table A5-2), and of the approaches to exposure assessment (Table A5-3). Although the results are divided into studies of childhood and adult cancers, the summaries of the methods used include both types of studies because the study designs are similar.

At the time these tables were constructed, 12 studies provided relevant data on childhood leukemia and five provided data on adult cancers. Eleven were conducted in the United States or western Europe, and the majority were published between 1986 and 1993 (Table A5-1). All but two of the reports concerned case-control studies, most of which were based on a comprehensive case ascertainment in a geographically defined population. Exposure assessment was based on some form of coding derived from the physical characteristics and distances of nearby power lines and other electric constructions, with varying sophistication in the classification methods, and a number of studies included measurements of magnetic-field strengths in homes (Table A5-3).

Results of the epidemiologic studies are organized into tables that focus on childhood leukemia (Table A5-4), childhood brain tumors (Table A5-5), childhood lymphoma (Table A5-6), other childhood cancers (Table A5-7), childhood cancer in the aggregate (Table A5-8), cohort studies of residential exposure and cancer including all ages (Table A5-9), adult leukemia (Table A5-10), and adult cancers generally (Table A5-11). In each table, the numbers of cases and controls in each group are provided along with the crude and adjusted odds ratios (or other measures of relative risk) with 95% confidence intervals, and the confounders that were considered are noted. The goal in presenting the tables was to provide sufficient information to help readers understand the rationale behind the committee's interpretation and to allow readers to draw their own conclusions.

A decision was made early in the committee's deliberations that the body



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