Table 4. Characteristics of study populations.

Analysis period		9mth-3yr	3-5yr	5-7yr
Total number of children		8408	6970	6747
Child	Exact age at outcome, mean years	3.12	5.21	7.22
Girl, %		49.0	49.6	49.3
Never breast-fed, %		26.3	25.3	28.0
Developmental delay at 9 months, %		13.1	12.8	13.3
Temperament problems at 9 months, %		12.4	12.3	12.7
Long-standing limiting illness at outcome, %		2.5	4.6	5.3
Father	Age at birth of child, mean years	32	32	32
Low educational attainment (NVQ equivalent level 1 or less) at 9 months, %		15.6	15.1	15.3
Not working at exposure, %		7.0	6.0	6.5
Symptoms of depression at exposure, %		8.3	5.9	6.2
Mother	Age at birth of child, mean years	30	30	30
Low educational attainment (NVQ equivalent level 1 or less) at 9 months, %		13.6	13.3	14.2
Long-standing limiting illness at exposure, %		8.2	7.7	12.1
Symptoms of depression at exposure, %		11.3	6.8	6.9
Household
Child had 2+ siblings in household at 9 months, %		20.2	19.9	20.4
Low occupational socioeconomic status (3-class NS-SEC 3) at exposure, %		22.0	19.3	18.9
Number of siblings in household changed during period, %		29.6	20.9	13.1
Birth father not resident at outcome, %		6.8	6.3	5.5

Weighted % of combined study population of boys and girls, excluding missing values. Full-term singleton births in families where the birth father was resident and interviewed at exposure, the family was English-speaking at outcome, and the birth mother gave the ‘main carer’ interview at exposure and outcome.