Frequencies of variables used as confounders
| Confounder | Number with data available | Frequency |
| Pre-term delivery | 12 935 | 5.3% <37 weeks |
| Sex of child | 14 676 | 51.8% male |
| Parity of mother | 12 787 | 48.2% primiparous |
| Whether breast fed at 7 days | 11 880 | 66.1% |
| Maternal depression at 2 months | 11 213 | 8.8% |
| Maternal education level (five levels) | 12 370 | From 20.8% lowest to 12.8% highest |
| Maternal smoking at 18 weeks gestation | 13 274 | 19.6% |
| Maternal locus of control | 12 471 | 45.8% external |
| Child’s exposure to environmental tobacco smoke at 15 months | 11 073 | 42.0% exposed |
| Attended crèche or equivalent by 3 years | 10 038 | 36.3% exposed |