Relationships between cigarette and e-cigarette use: (A) cross-sectional relationships between baseline cigarette and e-cigarette use; (B) prospective relationships between cigarette use at 1-year follow-up and e-cigarette use at baseline among baseline never used cigarettes; (C) prospective relationships between cigarette use at 1-year follow-up and e-cigarette use at baseline among baseline used once or used to use cigarettes
| Cigarette Use | Baseline e-cigarette use | |||
| Never | Tried | Infrequent | Frequent | |
| (1–2 times) | (1/month–1/week) | (>1/week) | ||
| n (%) | n (%) | n (%) | n (%) | |
| A. Cross-sectional relationships at baseline (n=2836) | ||||
| Never | 1743 (61.5) | 407 (14.4) | 40 (1.4) | 6 (0.2) |
| Once | 90 (3.2) | 201 (7.1) | 57 (2.0) | 10 (0.4) |
| Used to | 20 (0.7) | 59 (2.1) | 38 (1.3) | 22 (0.8) |
| Rarely (<1/week) | 8 (0.3) | 15 (0.5) | 31 (1.1) | 19 (0.7) |
| Occasional (1–6/week) | 1 (0.0) | 6 (0.2) | 20 (0.7) | 10 (0.4) |
| Frequent (>6/week) | 5 (0.2) | 7 (0.2) | 6 (0.2) | 15 (0.5) |
| B. Longitudinal relationships for baseline never users of cigarettes (n=1726) | ||||
| Never | 1259 (72.9) | 211 (12.2) | 13 (0.8) | 1 (0.1) |
| Once | 86 (5.0) | 65 (3.8) | 8 (0.5) | 0 (0.0) |
| Used to smoke | 19 (1.1) | 19 (1.1) | 1 (0.1) | 1 (0.1) |
| Rarely (<1/week) | 11 (0.6) | 12 (0.7) | 1 (0.1) | 0 (0.0) |
| Occasional (1–6/week) | 5 (0.3) | 3 (0.2) | 2 (0.1) | 0 (0.0) |
| Frequent (>6/week) | 3 (0.2) | 1 (0.1) | 3 (0.2) | 2 (0.1) |
| C. Longitudinal relationships for baseline triers of cigarettes (n=318) | ||||
| No change | 61 (19.2) | 131 (41.2) | 43 (13.5) | 14 (4.4) |
| Escalation | 9 (2.8) | 38 (11.9) | 17 (5.3) | 5 (1.6) |