Based on model fit statistics and parsimony, a five class trajectory model was chosen. Although the adjusted Lo-Mendell-Rubin test suggested retaining a six-class model, there was no improvement in the BIC for the six-class model, and the average posterior probabilities dropped below 0.70 for two of the classes, suggesting a loss in classification quality. While the BIC was slightly higher for the four-class model than for the five-class model, this difference was minimal and represented virtually no improvement in model fit. Additionally, the Lo-Mendell-Ruben test for the five-class model was significant at p<.05, providing additional evidence for retaining the five-class model. Finally, the five classes differed significantly in their smoking behavior, providing substantive evidence in support of a five-class solution.
Average latent class trajectories showing the mean estimated probability of smoking at each time point for the five distinct groups of smokers are shown in . A small number of “lifetime users” (n=11; average posterior probability = 0.94) maintained tobacco use across their lifespan. “Late quitters” (n=48; average posterior probability = 0.71) smoked until a median age of 69. Tobacco use for “middle quitters” (n=69; average posterior probability = 0.85) declined steadily and they quit at a median age of 56. “Early quitters” (n=26; average posterior probability = 0.70) ceased daily smoking sooner than other groups (i.e. median age of 40). For “low users” (n=78; average posterior probability = 0.95), tobacco use remained minimal across measurements.
summarizes disease outcomes for the different trajectory classes. Differences between groups were observed only for lung disease. Post-hoc paired comparisons showed that late quitters had a higher prevalence of lung disease than low users and early quitters. Further, middle quitters had a higher rate of lung disease than early quitters. No differences in rates of the remaining disease categories were found among the trajectory classes.
Chronic disease by smoking trajectory n (%)
Results of the discrete time survival latent class growth analysis indicated that, compared to low users and early quitters, middle and late quitters were less likely to survive to an advanced age. There were no significant differences in hazard rates between low users and early quitters and between middle and late quitters. Estimated survival curves () suggested that low users had a higher probability of survival by age 81–85, whereas late quitters had the lowest survival probability by age 81–85. Survival curves were similar for early and middle quitters, but by age 65–69, middle quitters had a significantly lower survival probability. Though lifetime users had an unexpectedly high survival probability in comparison to other groups, the fact that this class contained only 11 participants increases the likelihood that this result is not representative of this group of smokers.