state of birth at the time of their death or census). Finally, to quantitate the degree of error introduced into the exposure variable by the proxy approach, we culled 18–20 year old respondents from the 1980 and 1990 censuses to examine the percentage of those who lived in their birth states, and the percentages of those whose exposure would be correctly assigned under the assumption that they lived in their birth states, regardless of whether they had moved.
