Table 1 Descriptive statistics of confounding variables
Child variablesSample available to this studyWhole ALSPAC cohort
Gender (% male)49.947.8*
Ethnicity (% white)94.477.6*
Birth weight (mean, kg)3.43.3*
Twin (%)1.11.2
Birth order (mean, range)1.7, 1–141.8, 1–14*
Mother variables
Smoking in pregnancy (%)
    Never84.180.0*
    1–9 cigarettes/day6.57.7*
    More than 9 cigarettes/day9.512.2*
Education (%)
    Below secondary (below O level)21.925.4*
    Secondary (O level)39.936.9*
    Upper secondary (A level+)38.137.6
Mother’s SEG (%)
    I and II29.130.0
    III45.440.7*
    IV and V8.79.6
Teenage mothers (%)7.48.7*
Mother employed at 33 months (%)52.249.9*
Marital status at 47 months (%)
    Never married9.710.7
    Married once74.971.5*
    Separated/divorced)15.417.6*
Mother is vegetarian at birth (%)5.45.3
Family variables
Income at 33 months (%)
    <£20022.023.9*
    £200–£40050.244.9*
    >£40019.321.7*
% owning home83.078.4*
Parenting variables
Mother’s teaching practices score at 18 months (mean, range)8.0, 0–108.0, 0–10
Mother’s interest in child, score at 18 months (mean, range)32.3, 11–3632.2, 9–36
Mother’s parenting score at 6 months (mean, range)10.5, 0–1210.4, 0–12
Mother’s parenting score at 18 months (mean, range)40.7, 13–5140.7, 6–51
Breastfeeding (%)
    Never22.724.7*
    Up to 6 months45.644.9
    Over 6 months31.830.3
HOME score (mean, range)8, 0–128, 0–12
Child watches children’s programmes 18–42 months (%)72.759.5*

  • Asterisk indicates statistical significance difference in means at 1% level.

  • Sample sizes depend on missing information from individual variables. Sample size for available sample fluctuates between 5301 for mothers’ employment and 5741 for gender. Sample size for whole ALSPAC cohort fluctuates between 8515 for mother’s employment and 12 234 for gender and ethnicity.

  • ALSPAC, Avon Longitudinal Study of Parents and Children; SEG, socioeconomic group.