Table 4

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

PredictorsBaseline never used cigarettesBaseline once or used to use cigarettes
OR (95% CI)p ValueOR (95% CI)p Value
Model one without covariates
 Never used e-cigarettes1.001.00
 Ever used e-cigarettes5.38 (4.02 to 7.22)<0.0012.16 (1.01 to 4.62)0.046
Model two with covariates
 Never used e-cigarettes1.001.00
 Ever used e-cigarettes4.06 (2.94 to 5.60)<0.0011.89 (0.82 to 4.33)0.13
 Friend smokers = none1.001.00
 Friend smokers = a few1.87 (1.35 to 2.58)<0.0011.15 (0.50 to 2.66)0.75
 Friend smokers = most2.99 (1.52 to 5.87)0.0013.23 (1.19 to 8.77)0.022
 Male1.001.00
 Female1.32 (0.97 to 1.79)0.080.83 (0.45 to 1.52)0.55
 Family smokers = none1.001.00
 Family smokers = one0.76 (0.51 to 1.13)0.181.69 (0.61 to 4.68)0.31
 Family smokers = two2.05 (1.37 to 3.06)<0.0011.41 (0.48 to 4.12)0.53
 Family smokers = three or more1.90 (1.23 to 2.94)0.0041.23 (0.45 to 3.41)0.69
 Intentions0.70 (0.52 to 0.96)0.031.50 (0.87 to 2.57)0.14
 Attitudes0.68 (0.44 to 1.04)0.080.51 (0.28 to 0.90)0.020
 Norms0.89 (0.57 to 1.39)0.611.12 (0.56 to 2.23)0.75
 Perceived behavioural control1.00 (0.73 to 1.37)0.990.99 (0.58 to 1.69)0.96
 Self-efficacy1.09 (0.75 to 1.57)0.660.57 (0.35 to 0.94)0.027
 Free school meals0.99 (0.97 to 1.02)0.601.01 (0.96 to 1.07)0.62
Model three with covariates and interactions
 Never used e-cigarettes and Friend smokers = none1.00
 Ever used e-cigarettes and friend smokers = none7.74 (4.68—12.79)<0.001
 Never used e-cigarettes and Friend smokers = a few2.57 (1.72 to 3.84)<0.001
 Ever used e-cigarettes and friend smokers = a few7.84 (5.08–12.09)<0.001
 Never used e-cigarettes and friend smokers = most6.32 (2.68 to 14.91)<0.001
 Ever used e-cigarettes and friend smokers = most8.75 (3.68–20.83)<0.001
 Male1.00
 Female1.37 (1.01 to 1.86)0.04
 Family smokers = none1.00
 Family smokers = one0.76 (0.51 to 1.14)0.19
 Family smokers = two2.02 (1.35 to 3.03)<0.001
 Family smokers = three or more1.87 (1.21 to 2.90)0.005
 Intentions0.70 (0.52 to 0.96)0.03
 Attitudes0.67 (0.44 to 1.01)0.06
 Norms0.91 (0.59 to 1.41)0.69
 Perceived behavioural control1.00 (0.73 to 1.37)0.99
 Self-efficacy1.09 (0.75 to 1.59)0.65
 Free school meals0.99 (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.