Table 2.

Linear regression analysis of smoking heaviness at age 15 and general cognitive ability at age 15, before and after adjustment for potential confounders in ALSPAC (N = 2107)

Model	β coefficient	95% CI	p value
1	−1.362	−1.773 to −0.950	<0.001
2	−0.882	−1.220 to −0.544	<0.001
3	−0.721	−1.059 to −0.384	<0.001
4	−0.674	−1.021 to −0.327	<0.001
5a	−0.728	−1.199 to −0.257	0.002
5b	−0.722	−1.139 to −0.304	0.001
5c	−0.618	−1.007 to −0.229	0.002
6	−0.736	−1.238 to −0.233	0.004

Model 1 – Standardised general cognitive score at age 15 by a unit increase of 5-level smoking heaviness category at age 15; Model 2 – as model 1 with additional adjustment for general cognitive ability at age 8; Model 3 – as model 2 with additional adjustment for pre-birth confounders (gender, maternal education, maternal smoking, alcohol and cannabis use during pregnancy, maternal depression); Model 4 – as model 3 with additional adjustment for childhood confounders (truancy, hyperactivity, conduct problems, psychotic-like experiences, depression); Model 5a – as model 4 with additional adjustment for cannabis use at age 15; Model 5b – as model 4 with additional adjustment for alcohol use at age 15; Model 5c – as model 4 with additional adjustment for illicit drug use at age 15; Model 6 – as model 4 with additional adjustment for cannabis, alcohol and other illicit drug use at age 15.