We considered those who were permitted to drink legally prior to age 21 as having been “exposed” to permissive MLDA. Those who were not permitted to drink legally until age 21 were considered “unexposed”. To precisely assess exposure, we would need to know where individuals resided between the ages of 18 and 21. However, death and census records contain only state of birth and state or residence at time of observation. Therefore, we used state of birth as a proxy for state of residence during the ages of interest. Several tests were conducted to examine the validity of the proxy approach and the robustness of results to alternative approaches. First, we used state-of-residence at time of death or census as the proxy, rather than state-of-birth. Second, we conducted analyses that were limited to “likely non-movers” (individuals who resided in their 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
