Table 2b Summary of exposure/outcome/confounder data available on the 4750 used for analysis—categorical variables

	Categories	N	%
Exposures
Maternal smoking in pregnancy	No	3503	81.5
	Yes	794	18.5
Paternal smoking in pregnancy	No	3106	69.0
	Yes	1694	31.0
Breastfeeding	Exclusive	1604	36.4
	Partial	2182	49.5
	None	621	14.1
Age at introduction of solids	⩽2 months	598	13.3
	3 months	2572	57.3
	⩾4 months	1317	29.4
Time spent in the car at 38 months	<4 h per week	2957	68.3
	4–5 hours per week	846	19.5
	>5 h per week	526	12.2
Time spent watching television at 38 months	⩽4 h per week	1011	23.4
	4.1–8 h per week	2072	47.9
	>8 h per week	1242	28.7
Frequency of playing computer games at 57 months	Rarely/not at all	1755	41.5
	Once a month	736	17.4
	Once a week	959	22.7
	2–7 times per week	776	18.4
Confounders
Parity	0	2277	49.2
	1	1599	34.6
	⩾2	750	16.2
Social class in pregnancy	V	121	2.7
	IV	565	12.7
	III manual	1123	25.2
	III non-manual	1235	27.7
	II	1218	27.4
	I	190	4.3
Maternal education	None/CSE	511	11.1
	Vocational	361	7.8
	O levels	1604	34.7
	A levels	1313	28.4
	Degree	837	18.1
Gender	Male	2267	47.7
	Female	2483	52.3
Pubertal status at age 15 years	Stage I/II/III	180	4.4
	Stage IV	1691	41.1
	Stage V	2240	54.5^a

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