First, a baseline Markov model was used to describe the prevalence of past month smoking stages at Gemcitabine hydrochloride each assessment and to describe the rates of transitions between stages across adjacent timepoints. Second, to take into account smoking history prior to the 12th grade, we added early age at initiation (prior to high school) in the model to predict baseline stage membership (12th grade) and stage transitions. Finally, gender, college status, and binge drinking were added separately to the model to predict 12th-grade stage and transitions. All models were estimated using PROC LTA, a new SAS procedure for latent transition analysis developed by the Methodology Center at Penn State for SAS version 9.1 for Windows. An introduction to a general modeling approach for latent transition analysis with grouping variables and covariates is provided by Lanza and Collins (2008).

Results Baseline model results Table 1 presents the prevalence rates of smoking stages at each assessment from 12th grade until S2. The prevalence rates are the proportions of individuals in each smoking stage at each assessment. The proportion of nonsmokers declined slightly from 65% in the 12th grade to 60% at F2. The proportion of light and intermittent smokers was relatively consistent over time, at about 19%. The prevalence of heavy smoking increased slightly over time from 16% in the 12th grade to 21% at S2. Table 1. Prevalence rates of past month smoking stages and transition rates between past month smoking stages (N=990) Transition rates also are shown in Table 1.

The transition rates are the probabilities of smoking stage membership at time t + 1 conditional on smoking stage membership at time t. There is one set of transition rates for each pair of adjacent times; that is, there is a set of rates for the transitions from 12th grade to F1, a set from F1 to S1, etc. Whereas the movement from nonsmoking to light and intermittent smoking was consistent over time, the largest movement from light and intermittent GSK-3 to heavy smoking occurred during the transition out of high school (12th grade to F1). In contrast, the highest stabilities for all three types of smoking were seen from F1 to S1. The probability of light and intermittent smokers remaining light and intermittent smokers across adjacent timepoints varied over time, ranging from 56% between F2 and S2 to 72% between F1 and S1. In contrast, nonsmokers (89%�C91%) and heavy smokers (75%�C90%) had higher stability over time.