Residential exposure to magnetic fields: an empirical examination of alternative measurement strategies

Objectives-To investigate the impact of measuring a single home then imputi ng information from another home among subjects who lived in two homes in a subset of the National Cancer Institute/Children's Cancer Group (NCI/CCG) investigation of residential exposure to magnetic fields and risk of childh ood leukaemia. Methods-Each subject's summary time weighted average (TWA) exposure was der ived from measurements or two homes, weighted by the fraction of the refere nce period lived in the residence. The three cost efficient field work stra tegies examined were measuring: (a) the longer lived in home; (b) the curre ntly lived in home; and (c) the former lived in home. Two different methods were used for imputing the missing values: (a) control mean imputation, (b ) status specific mean imputation. The subject's summary exposure to magnet ic fields estimated with each approach was compared with the subject's TWA calculated from measurements in both homes. The association between estimat ed exposure to magnetic fields and the risk of leukaemia under different ap proaches was examined with unconditional logistic regression analysis. Results-The Pearson correlation coefficient between the two measurements wi thin subjects was 0.31 (p<10(-4)), indicating a lack of independence of mea surements. Differences were found between mean exposures in current and for mer homes of cases, and between longer and shorter lived in homes of contro ls. All methods with measurements from one of the homes in conjunction with imputation of measurements for the second home led to marked attenuation o f risk estimates at the highest exposure category, particularly when measur ements from current homes were used and those from former homes were impute d. Conclusion-Results argue against attempting to estimate lifetime magnetic f ield exposure from imputed values derived from current residences to fill i n gaps caused by unmeasured residences previously lived in.