Table 2.

Youth e-cigarette use and ever cigarette smoking initiation: Baseline-adjusted and marginal structural model discrete-time regression results

			Crudea		Unweighted Baseline- Adjustedb		MSMb,c
Time-varying e-cigarette use	No. youth initiating smoking	Total person- waves	RR	95% CI	RR	95% CI	RR	95% CI
Never use	1,001	20,368	1.0	REF	1.0	REF	1.0	REF
Ever use	562	2,799	3.8	3.4-4.2	2.7	2.4-3.0	2.4	2.1-2.7
Former used	378	2,101	3.4	3.0-3.8	2.5	2.2-2.9	2.2	2.0-2.5
Current usee	184	698	4.9	4.2-5.8	3.5	2.9-4.1	3.1	2.6-3.7
Tobacco flavored	41	241	3.5	2.6-4.8	2.5	1.8-3.5	2.4	1.7-3.3
Non-Tobacco flavored	521	2,558	3.8	3.4-4.2	2.8	2.5-3.1	2.4	2.2-2.7
E-cigarette frequencyf
0 days	399	2,204	1.0	REF	1.0	REF	1.0	REF
1-2 days	72	315	1.2	0.96-1.6	1.2	0.92-1.5	1.2	0.93-1.6
≥3 days	91	280	1.8	1.4-2.3	1.7	1.3-2.1	1.8	1.4-2.2

CI indicates confidence interval, MSM marginal structural model, RR risk ratio.

^aModel includes time-varying exposure and time-specific intercept

^bModel includes time-varying exposure, time-specific intercept, and wave 1 values of age, sex, race/ethnicity, parental education, living with tobacco user, grades, externalizing mental health, alcohol, marijuana, and other tobacco use (past 12-months), cigarette susceptibility, perceived harm of vaping, having a favorite tobacco ad.

^cWeighted by inverse probability of treatment and censoring weights to adjust for bias due to time-dependent confounding and selection bias due to lost to follow-up.

^dFormer use defined as ever using e-cigarettes, but not in the past 30-days at the wave immediately prior to the outcome.

^eCurrent use defined as ever using e-cigarettes and used in the past 30-days at the wave immediately prior to the outcome.

^fFrequency defined as the number of days e-cigarette were used in the past 30-days at the wave immediately prior to the outcome. Models restricted to youth who ever initiated e-cigarettes.