Table 4 Effect modification of a priori considered variables: regression coefficients (beta) and 95% confidence intervals (95% CI) from stratified linear regression models adjusted for age, gender, height and weight

	No	FEV₁ (ml)	p Value*	FEF_25–75 (ml/s)	p Value*
	No	Beta (95% CI)	p Value*	Beta (95% CI)	p Value*
Gender
Boys	711	−4 (−29 to 21)	0.250	−26 (−81 to 29)	0.056
Girls	648	−23 (−49 to 2)	0.250	−103 (−163 to −43)	0.056
Age (years)
9–10	1213	−6 (−25 to 12)	0.011	−53 (−95 to −11)	0.014
11–14	146	−89 (−154 to −24)	0.011	−158 (−313 to −3)	0.014
Maternal education
Low	538	3 (−27 to 33)	0.208	−9 (−75 to 57)	0.061
High	801	−23 (−46 to 0)	0.208	−91 (−144 to −38)	0.061
ETS at home
No	683	−13 (−38 to 11)	0.907	−29 (−86 to 29)	0.080
Yes	662	−13 (−39 to 13)	0.907	−99 (−156 to −42)	0.080
Fruit intake
0–2 times/week	377	−5 (−41 to 31)	0.634	−109 (−190 to −28)	0.188
>2 times/week	982	−15 (−36 to 5)	0.634	−47 (−94 to 1)	0.188
BMI
Low–normal	1489	−12 (−34 to 3)	0.382	−57 (−99 to −15)	0.322
High†	271	16 (−57 to 89)	0.382	−143 (−293 to 7)	0.322
Asthma
Never	1152	−16 (−36 to 4)	0.672	−67 (−113 to −21)	0.633
Ever	194	−5 (−46 to 37)	0.672	−41(−133 to 52)	0.633
Positivity to prick test
No	895	−13 (−34 to 8)	0.929	−40 (−87 to 8)	0.112
At least one	417	−14 (−49 to 21)	0.929	−119 (−197 to −40)	0.112

*p Value for interaction. †Highest quintile.
BMI, body mass index; ETS, environmental tobacco smoke; FEF_25–75, forced expiratory flow between 25% and 75% of FVC; FEV₁, forced expiratory volume in 1 s; FVC, forced vital capacity.