, 2004). The other five items in the NDSS scales did not exhibit significant change over time. As illustrated by Liu (2008), another advantage of applying the longitudinal IRT model is that it is not necessary that every subject inhibitor CHIR99021 responds to the complete set of items at each wave. In the analyses of an average score, the results are often based on the sample of subjects who respond to some minimum number of items (e.g., 50% or 75%). In this case, the degree of certainty/uncertainty in calculation of this average varies (because it is based on different numbers of item response), but is ignored in the analysis. However, the longitudinal IRT model employed in this study allows for different number of item responses at different time points and/or for different subjects, and accounts for these differences in terms of the model standard errors.

The present study also adds to our understanding of the development of nicotine dependence among adolescents. Most prior work has focused on examining the dimensional structure of nicotine dependence in adolescents (e.g., Clark, Wood, Martin, Cornelius, Lynch, & Shiffman, 2005) or examining how symptoms may change over time following treatment (e.g., Strong et al., 2007). One prior study has also examined the predictive validity of specific symptoms of nicotine dependence among very light, infrequent adolescent smokers (Dierker & Mermelstein, 2010). We found a fair amount of heterogeneity in level of symptoms over time, but our results suggest that some symptoms may be especially important to track to help with the early identification of adolescents who are vulnerable to developing higher levels of dependency.

These symptoms include ones that reflect more drive toward smoking, compared with those focused on relief of withdrawal. The drive dimension, among these adolescent very light smokers, may reflect a vulnerability to escalate and to develop further dependence. The examination of nicotine dependence and changes in patterns of dependency over time helps to increase our understanding of the development of dependence and to identify potential screening items for adolescents at high risk for escalation. Funding This work was supported by National Cancer Institute grant 5PO1 CA98262. Declaration of Interests None.
Tobacco control policies have been shown to be effective in reducing smoking.

There is significant evidence that laws that restrict smoking in public places and workplaces result Cilengitide in less smoking (Bauer, Hyland, Li, Steger, & Cummings, 2005; Fichtenberg & Glantz, 2002; Hahn et al., 2008; Moskowitz, Lin, & Hudes, 2000), and policy-based approaches have been shown to reduce youth tobacco use (Forster, Widome, & Bernat, 2007). Despite these successes, there is not always widespread support for tobacco control policies. Poland et al.