Population coverage and nonresponse bias in a large-scale human exposure study.

We used estimates derived from screener variables of the National Human Exposure Assessment Survey (NHEXAS) Phase I field study in EPA Region V (one of three NHEXAS Phase I field studies) to examine biases resulting from survey nonresponse and/or incomplete population coverage inherent in the study design. For variables with population values obtainable from Census projections, the combined effect of nonresponse and coverage bias was tested for after each stage of nonresponse using design-based weights. For variables where population values were not available as Census projections, nonresponse bias was tested for after the screener stage of nonresponse using weights adjusted for screener nonresponse. Additional tests for bias were performed using final survey weights to evaluate the performance of survey weight adjustments in reducing observed bias. Comparison of biases estimated using both design-based and adjusted weights was used to identify potentially important weight adjustment variables for future exposure studies, identify possible weaknesses in survey design strategies, and support the use of nonresponse and poststratification weight adjustments to reduce bias in future survey studies.