As is common for such measures (Yamaguchi, 1991), exact onset times could not be determined. For individuals who reported to have drunk alcohol by the first wave, data were left-censored; for those who did not report any drinking up to the last wave in which they responded, data were right-censored. Other responses were interval-censored, as the time of onset could only be narrowed to a 30-day period prior to an assessment. Further uncertainty was introduced by the fact that our survey did not ask about drinking during the period from the previous assessment up to just before the 30 day current assessment window. Additionally, because most participants (48.6%) missed one or more surveys for which they were eligible (i.e., enrolled in school and not opted out of the study), it is possible that some would have reported onset on that survey, again producing an incorrectly delayed time of onset. Although a more comprehensive measure of onset would have been ideal, this procedure would likely give rise to a bias only in estimating the absolute rate of onset. However, our proportional
