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—We concur with Dr Kirkland that subgroups of exposure are of less interest than total exposure to ETS and, as noted in our report, some exposures are more likely to be accurately recalled and reported than others. However, to estimate the risk of exposure to ETS, it is necessary to ask specific questions about exposures in a variety of settings during different time periods and then to summarize those data as we did in Table 5 for all childhood exposures combined and in Tables 7 and 8 for all adulthood exposures combined. The study was designed to collect data on a finite number of subgroups and to combine the subgroup data to categorize study subjects into levels of exposure ranging from little or none to lengthy. The unit of exposure was "years," which does not presume a level of precision that is unachievable (eg, hours per day per
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