Data were analyzed using SAS version 9.1 (SAS Institute Inc, Cary, NC). Differences in blood measures (total and free parent, and fP) and regional β2*-nAChR availability, VT/fP, between subjects in the nonsmoker control group and each of the 1 day, 1 week, 2 week, 4 week, and 6-12 week abstinent smoker groups were assessed using two-sample t-tests. Differences in regional brain VT/fP and behavioral measures of tobacco smoking and withdrawal between subjects in the abstinent smoker groups were assessed using repeated measures, mixed-effects regression models with group as a fixed effect and compound symmetry covariance structure across repeated measurements. These between group differences were evaluated using repeated measures, mixed-effects models in order to account for the observations between abstinent smoker groups that were not entirely independent, given that some abstinent smokers contribute observations to more than one group. For models examining between-group differences for which there was a significant effect of group, four planned post hoc between-group comparisons were conducted, i.e., between 1 week abstinent smokers and 1 day, 2 week, 4 week, and 6-12 week abstinent smokers. For these
