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Trajectories of Smoking Among College Freshmen:
Data from the UpTERN Study
Brian R. Flay, Eisuke Segawa, Donald Hedeker, Craig Colder
and TERN members
Statistical Model: Hybrid of HLM & GMM
• In pure GMM, classification is dominated by intercepts
• Our interest was more in changes over time, or slopes
• Therefore, we specified a common (random-effect) intercept model
• Model estimates only one intercept (mean over all subjects), but it provides intercept estimates for each subject as in HLM
Overall Class 1 Class 2 Class 3 Class 4 Class 5 Class 6 Class 7
Mean Intercept 2.394
Slope 0.516 0.207 0.001 -0.177 -0.278 -0.629 -1.145
Quadratic 0.127 0.068 0.015 0.000 0.056 0.198 0.194
4.00 1.6 0.0 -1.4 -2.2 -4.9 -8.9
Variance Intercept 9.2
Slope Fixed Fixed Fixed Fixed Fixed Fixed Fixed
Quadratic Fixed Fixed Fixed Fixed Fixed Fixed Fixed
Int X slp Fixed Fixed Fixed Fixed Fixed Fixed Fixed
Y1- y9 3.9 0.3 0.2 2.1 0.4 1.0 13.3
Covarianceyi with yj 0.9 0.0 0.0 0.7 0.1 0.45 4.9
10% 5% 18% 22% 17% 12% 16%
20 10 34 43 32 24 30
Yellow cells statistically significant BIC = 6574.1
Change in cigs/ day
Proportion of class
Number of subjects
Common Random Intercept GMM for non-centered data: 7 Class Model
Class
Change Subjects Subjects
Class1 Increasers 1 5% 19 95% 20
Class2Low-level
Increasers 0 0% 10 100% 10
Class3Occasional Smokers 19 56% 15 44% 34
Class4 Chippers 30 70% 13 30% 43
Class5Reducing Chippers 32 100% 0 0% 32
Class6High Reducing
Chippers 24 100% 0 0% 24
Class7High Variation
Decreasers 29 97% 1 3% 30
Classification by increase or decreasein smoking during the year
9 month+follow up
Ave
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7class 1
9 month+follow up
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0 5 10 15 20
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7class 2
9 month+follow up
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0 5 10 15 20
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81
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7class 3
9 month+follow up
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0 5 10 15 20
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7class 4
What happened to these groups the following year?
What happened to these groups the following year?
9 month+follow up
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7class 5
9 month+follow up
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0 5 10 15 20
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7class 6
9 month+follow up
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0 5 10 15 20
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7class 7
Conclusions• The overall levels of smoking among
college students are lower than expected
• Nevertheless, some students increased their rates of smoking and others decreased
• Hybrid HLM and Growth Mixture Analysis provided a meaningful grouping of students that described their patterns of smoking during freshman year
• All smokers are at risk of increasing or maintaining their smoking levels!