Table 2.

Negative binomial regression analysis for the depression score outcome (at the age of 17) by smoking behavior (at the age of 14)

Cigarette smokinga	Adjusted for age and sex			Adjusted for all covariates			Adjusted for all covariates and pre-existing depressiveness
	IRR	95% CI	p	IRR	95% CI	p	IRR	95% CI	p
Lifetime cigarettes smoked	N = 4110			N = 4110			N = 3923
1–50	1.19	1.11–1.27	2.5e-07	1.09	1.01–1.17	0.021	1.08	1.01–1.17	0.032
> 50	1.43	1.28–1.60	7.1e-10	1.12	0.97–1.29	0.121	1.17	1.01–1.35	0.037
Current smoking status at the age of 14	N = 4152			N = 4152			N = 3960
Experimenters	1.16	1.08–1.25	2.2e-05	1.10	1.02–1.18	0.012	1.09	1.01–1.18	0.021
Quitters or trying to quit	1.31	1.14–1.51	1.8e-04	1.12	0.96–1.29	0.143	1.11	0.96–1.29	0.160
Regular smokers	1.46	1.32–1.62	8.1e-14	1.16	1.01–1.32	0.031	1.19	1.03–1.36	0.015

^aReference category: never smokers

IRR, incidence rate ratio; CI, confidence interval

All covariates: age, sex, school grades, drinking alcohol to intoxication, health status, family structure, parental smoking status, and parental education