Table 1.

Odds ratios of linear and quadratic models for ever smoked and regular smoking between 1998–2015 among students in England, Scotland and Wales

		Ever smoked				Regular smoking
		Linear	P value	Quadratic	P value	Linear	P value	Quadratic	P value
Whole sample (n=242 855; 243 111)	Year	0.91 (0.91 to 0.91)	<0.001	0.95 (0.93 to 0.96)	<0.001	0.93 (0.93 to 0.94)	<0.001	0.98 (0.96 to 1.00)	0.025
	Year2	–	–	1.00 (1.00 to 1.00)	<0.001	–	–	1.00 (1.00 to 1.00)	<0.001
	Level	0.89 (0.84 to 0.95)	<0.001	0.89 (0.84 to 0.95)	<0.001	0.79 (0.71 to 0.88)	<0.001	0.80 (0.72 to 0.90)	<0.001
	Post-slope	0.97 (0.96 to 0.99)	<0.001	1.01 (0.99 to 1.03)	0.231	0.98 (0.96 to 1.01)	0.231	1.04 (1.00 to 1.08)	0.028
Male only subgroup (n=121 879; 122 042)	Year	0.92 (0.92 to 0.93)	<0.001	0.93 (0.91 to 0.95)	<0.001	0.94 (0.94 to 0.95)	<0.001	0.95 (0.92 to 0.98)	<0.001
	Year2	–	–	1.00 (1.00 to 1.00)	0.621	–	–	1.00 (1.00 to 1.00)	0.583
	Level	0.88 (0.81 to 0.96)	0.005	0.88 (0.81 to 0.96)	0.005	0.83 (0.70 to 0.97)	0.022	0.83 (0.70 to 0.97)	0.023
	Post-slope	0.98 (0.95 to 1.00)	0.034	0.98 (0.95 to 1.01)	0.212	1.00 (0.96 to 1.04)	0.921	1.01 (0.96 to 1.06)	0.794
Female only subgroup (n=120 976; 121 069)	Year	0.90 (0.89 to 0.90)	<0.001	0.96 (0.94 to 0.98)	<0.001	0.92 (0.92 to 0.93)	<0.001	1.00 (0.98 to 1.03)	0.922
	Year2	–	–	1.00 (0.99 to 1.00)	<0.001	–	–	0.99 (0.99 to 1.00)	<0.001
	Level	0.90 (0.83 to 0.98)	0.014	0.90 (0.83 to 0.98)	0.018	0.76 (0.65 to 0.89)	<0.001	0.78 (0.67 to 0.91)	0.002
	Post–slope	0.97 (0.95 to 0.99)	0.009	1.05 (1.01 to 1.08)	0.003	0.97 (0.93 to 1.01)	0.139	1.07 (1.02 to 1.12)	0.009
13 year olds only (n=126 960; 127 100)	Year	0.89 (0.89 to 0.90)	<0.001	0.96 (0.94 to 0.98)	<0.001	0.91 (0.90 to 0.92)	<0.001	1.02 (0.98 to 1.05)	0.378
	Year2	–	–	1.00 (0.99 to 1.00)	<0.001	–	–	0.99 (0.99 to 1.00)	<0.001
	Level	0.82 (0.74 to 0.91)	<0.001	0.83 (0.75 to 0.92)	<0.001	0.73 (0.57 to 0.92)	0.009	0.76 (0.60 to 0.97)	0.027
	Post-slope	0.99 (0.96 to 1.01)	0.304	1.07 (1.03 to 1.10)	<0.001	1.00 (0.95 to 1.07)	0.902	1.14 (1.06 to 1.23)	<0.001
15 year olds only (n=115 895; 116 011)	Year	0.93 (0.93 to 0.93)	<0.001	0.94 (0.92 to 0.95)	<0.001	0.94 (0.93 to 0.94)	<0.001	0.97 (0.95 to 0.99)	0.003
	Year2	–	–	1.00 (1.00 to 1.00)	0.390	–	–	1.00 (1.00 to 1.00)	0.009
	Level	0.92 (0.85 to 0.99)	0.035	0.92 (0.85 to 0.99)	0.036	0.79 (0.70 to 0.90)	<0.001	0.80 (0.70 to 0.90)	<0.001
	Post-slope	0.96 (0.94 to 0.98)	<0.001	0.96 (0.94 to 0.99)	0.012	0.98 (0.95 to 1.01)	0.240	1.01 (0.97 to 1.06)	0.497