Table 3

Prevalence of TTFC nicotine dependence stratified by a number of CPD among current smokers (The National Survey on Drug Use and Health, 2002–15, aged 12+)

Current smokers	2002*	2003	2004	2005	2006	2007	2008	2009	2010	2011	2012	2013	2014	2015	Linear trend(13-year increment)
Current smokers	Prevalence of TTFC (unweighted n, weighted %, weighted SE)														Unadjusted OR (95% CI)	T-test (p value)	Adjusted OR (95% CI)	T-test (p value)
Among current smokers
Total (n)	14 996	15 336	15 096	14 936	14 411	14 260	13 889	13 762	13 673	13 549	12 549	11 950	11 604	11 415
TTFC (n)	6419	6484	6378	6188	5906	5930	5745	5662	5556	5603	5256	4926	11 604	4973	0.89 (0.84,0.94)	t=−4.33 (0.0000)	1.18 (1.10,1.26)	t=4.90 (0.0000)
%	50.42	50.01	50.58	48.15	48.32	47.94	48.58	47.33	46.75	48.72	47.35	47.39	47.77	47.13
SE	0.68	0.67	0.87	0.84	0.85	0.62	0.79	0.78	0.87	0.78	0.75	0.66	0.64	0.7
No TTFC (n)	8577	8852	8718	8748	8505	8330	8144	8100	8117	7946	7293	7024	6527	6442
%	49.58	49.99	49.42	51.85	51.68	52.06	51.42	52.67	53.25	51.28	52.65	52.61	52.23	52.87
SE	0.68	0.67	0.87	0.84	0.85	0.62	0.79	0.78	0.87	0.78	0.75	0.66	0.64	0.7
Among current smokers
Stratified by CPD
1–5 CPD (n)	1177	1250	1247	1164	1106	1130	1201	1249	1242	1300	1282	1174	1213	1179	1.19 (1.08,1.32)	t=3.51 (0.0006)	1.20 (1.08,1.33)	t=3.53 (0.0005)
%	18.72	18.57	18.16	16.38	18.19	17.52	17.80	17.29	18.73	19.48	20.28	18.88	20.90	20.57
SE	0.76	0.92	0.98	0.83	0.8	0.81	0.83	0.7	0.76	0.98	1.01	0.84	0.73	0.85
6–15 CPD (n)	1942	1995	2033	2015	1973	2047	2017	1995	1995	2019	1908	1854	1849	1783	1.26 (1.14,1.39)	t=4.59 (0.0000)	1.30 (1.18,1.44)	t=5.25 (0.0000)
%	51.46	52.34	54.08	52.5	50.49	53.73	54.2	55.41	52.53	58.19	52.79	58.26	56.86	57.11
SE	1.35	1.32	1.46	1.27	1.49	1.49	1.30	1.78	1.49	1.65	1.56	1.32	1.49	1.42
16+ CPD (n)	3288	3231	3094	3000	2822	2748	2520	2417	2315	2280	2061	1893	2007	2009	1.03 (0.92,1.16)	t=0.55 (0.5838)	1.02 (0.91,1.15)	t=0.40 (0.6925)
%	80.77	80.72	80.68	78.01	79.12	78.02	80.05	79.3	81.39	80.28	81.43	80.65	80.9	79
SE	1.03	0.91	1.09	1.10	1.00	1.10	1.20	1.21	1.13	1.09	1.18	1.20	1.25	1.13
Differential time trend
Year × CPD															Wald F(2) =3.82 (0.0237)		Wald F(2)=4.04 (0.0194)

Logistic regressions were applied to examine the time trend within each particular group. An OR greater/less than 1 indicated a trend for the prevalence of TTFC nicotine dependence among each smoking category stratified by CPD to increase/decrease during 2002–2015, and t tests showed the significance of time trends.
Adjusted models—adjusted for number of CPD (1–5, 6–15, 16 or more), age (12–17 years old, 18–25 years old, 26 or older), gender (male, female) and family income (<$20 000, $20 000-$74 999, >$75 000).
Adjusted models stratified by CPD—adjusted for age (12–17 years old, 18–25 years old, 26 or older), gender (male, female) and family income (<$20 000, $20 000-$74 999, >$75 000).
All current smokers are combination of current daily and non-daily smokers.
*2002 as reference year.
CPD, number of cigarettes smoked per day in past 30 days; TTFC, the time to their first cigarette (TTFC<30 min) after waking in the morning.