The potential for bias due to attrition in the National Exposure Registry: an examination of reasons for nonresponse, nonrespondent characteristics, and the response rate.

This study examined attrition in the Trichloroethylene (TCE) Subregistry of the National Exposure Registry (NER). The analyses focused on 3915 persons exposed to the chemical TCE through the drinking water in their home. Baseline data were compared for subgroups of the TCE Subregistry members who were eligible to participate in the first TCE Subregistry follow-up. Study members were grouped according to their participation status in the first follow-up: remainers (n = 3494) and losses (n = 421), and three subgroups of losses: refusals, unable to locate, and unable to contact. The comparison of demographic variables of remainers and losses revealed that remainers had a higher percent of females, currently smoked less, were older, and fewer had no education and more had education beyond high school. These differences occurred for the losses subgroups unable to locate and unable to contact, however, not for refusals. The comparison of reporting rates of remainers and losses for 2.3 health outcomes revealed statistically significant decreases by losses for five health conditions but the pattern of statistically significant differences for the losses subgroups was not clear-cut. Altogether, the analyses indicated that the potential for bias due to attrition was minimal.