Trajectories of Smoking Among College Freshmen:

Data from the UpTERN Study

Brian R. Flay, Eisuke Segawa, Donald Hedeker, Craig Colder

and TERN members

Selection of Subjects for These Analyses

Fitted Curve for Standard Mixed Effect Model

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

7-class Common Random Intercept GMM

Class 1: Increasers

Class 2:Low-level increasers

Class 3: Occasional Smokers

Class 4: Regular Chippers

Class 5: Reducing Chippers

Class 6: Higher reducing chippers

Class 7: High variation decreasers

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

rag

e n

um

be

r o

f cig

are

tte p

er

da

y

0 5 10 15 20

02

46

81

01

2

7class 1

9 month+follow up

Ave

rag

e n

um

be

r o

f cig

are

tte p

er

da

y

0 5 10 15 20

02

46

81

01

2

7class 2

9 month+follow up

Ave

rag

e n

um

be

r o

f cig

are

tte p

er

da

y

0 5 10 15 20

02

46

81

01

2

7class 3

9 month+follow up

Ave

rag

e n

um

be

r o

f cig

are

tte p

er

da

y

0 5 10 15 20

02

46

81

01

2

7class 4

What happened to these groups the following year?

What happened to these groups the following year?

9 month+follow up

Ave

rag

e n

um

be

r o

f cig

are

tte

pe

r d

ay

0 5 10 15 20

02

46

81

01

2

7class 5

9 month+follow up

Ave

rag

e n

um

be

r o

f cig

are

tte

pe

r d

ay

0 5 10 15 20

02

46

81

01

2

7class 6

9 month+follow up

Ave

rag

e n

um

be

r o

f cig

are

tte

pe

r d

ay

0 5 10 15 20

02

46

81

01

2

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!