yielded significant improvement in model fit. An initial analysis was conducted without covariates in the model. Baseline levels of cotinine, FTND, age, cigarettes per day, and sex were then entered into the model for a second analysis. Because there was no change in results related to the covariates, the results of the initial analysis are reported here. We used a computer program, PROC MIXED (SAS Institute Inc., Cary, NC) to estimate and test the models.
