Association of baseline ever used e-cigarettes with ever used cigarettes at follow-up (among never users of cigarettes at baseline; n=1726; left-hand column) or increased use of cigarettes at follow-up (among baseline once or used to use cigarettes; n=318; right-hand column) controlling for clustering by school
| Predictors | Baseline never used cigarettes | Baseline once or used to use cigarettes | ||
| OR (95% CI) | p Value | OR (95% CI) | p Value | |
| Model one without covariates | ||||
| Never used e-cigarettes | 1.00 | 1.00 | ||
| Ever used e-cigarettes | 5.38 (4.02 to 7.22) | <0.001 | 2.16 (1.01 to 4.62) | 0.046 |
| Model two with covariates | ||||
| Never used e-cigarettes | 1.00 | 1.00 | ||
| Ever used e-cigarettes | 4.06 (2.94 to 5.60) | <0.001 | 1.89 (0.82 to 4.33) | 0.13 |
| Friend smokers = none | 1.00 | 1.00 | ||
| Friend smokers = a few | 1.87 (1.35 to 2.58) | <0.001 | 1.15 (0.50 to 2.66) | 0.75 |
| Friend smokers = most | 2.99 (1.52 to 5.87) | 0.001 | 3.23 (1.19 to 8.77) | 0.022 |
| Male | 1.00 | 1.00 | ||
| Female | 1.32 (0.97 to 1.79) | 0.08 | 0.83 (0.45 to 1.52) | 0.55 |
| Family smokers = none | 1.00 | 1.00 | ||
| Family smokers = one | 0.76 (0.51 to 1.13) | 0.18 | 1.69 (0.61 to 4.68) | 0.31 |
| Family smokers = two | 2.05 (1.37 to 3.06) | <0.001 | 1.41 (0.48 to 4.12) | 0.53 |
| Family smokers = three or more | 1.90 (1.23 to 2.94) | 0.004 | 1.23 (0.45 to 3.41) | 0.69 |
| Intentions | 0.70 (0.52 to 0.96) | 0.03 | 1.50 (0.87 to 2.57) | 0.14 |
| Attitudes | 0.68 (0.44 to 1.04) | 0.08 | 0.51 (0.28 to 0.90) | 0.020 |
| Norms | 0.89 (0.57 to 1.39) | 0.61 | 1.12 (0.56 to 2.23) | 0.75 |
| Perceived behavioural control | 1.00 (0.73 to 1.37) | 0.99 | 0.99 (0.58 to 1.69) | 0.96 |
| Self-efficacy | 1.09 (0.75 to 1.57) | 0.66 | 0.57 (0.35 to 0.94) | 0.027 |
| Free school meals | 0.99 (0.97 to 1.02) | 0.60 | 1.01 (0.96 to 1.07) | 0.62 |
| Model three with covariates and interactions | ||||
| Never used e-cigarettes and Friend smokers = none | 1.00 | |||
| Ever used e-cigarettes and friend smokers = none | 7.74 (4.68—12.79) | <0.001 | ||
| Never used e-cigarettes and Friend smokers = a few | 2.57 (1.72 to 3.84) | <0.001 | ||
| Ever used e-cigarettes and friend smokers = a few | 7.84 (5.08–12.09) | <0.001 | ||
| Never used e-cigarettes and friend smokers = most | 6.32 (2.68 to 14.91) | <0.001 | ||
| Ever used e-cigarettes and friend smokers = most | 8.75 (3.68–20.83) | <0.001 | ||
| Male | 1.00 | |||
| Female | 1.37 (1.01 to 1.86) | 0.04 | ||
| Family smokers = none | 1.00 | |||
| Family smokers = one | 0.76 (0.51 to 1.14) | 0.19 | ||
| Family smokers = two | 2.02 (1.35 to 3.03) | <0.001 | ||
| Family smokers = three or more | 1.87 (1.21 to 2.90) | 0.005 | ||
| Intentions | 0.70 (0.52 to 0.96) | 0.03 | ||
| Attitudes | 0.67 (0.44 to 1.01) | 0.06 | ||
| Norms | 0.91 (0.59 to 1.41) | 0.69 | ||
| Perceived behavioural control | 1.00 (0.73 to 1.37) | 0.99 | ||
| Self-efficacy | 1.09 (0.75 to 1.59) | 0.65 | ||
| Free school meals | 0.99 (0.96 to 1.02) | 0.47 | ||
Follow-up ever used cigarettes: model without covariates, AIC=1281.3; model with covariates, AIC=1226.5; model with covariates and interactions, AIC=1218.7; follow-up escalation of cigarette use: model without covariates, AIC=334.1; model with covariates, AIC=327.5.