Comparison of characteristics between participants with IQ measured at 8 or 15 years and those that did not have IQ measured¹.

1Abbreviations; BMI, body mass index; CSE, Certificate of Secondary Education; IQ, intelligence quotient; WASI, Wechsler Abbreviated Scale of Intelligence; WISC, Wechsler Intelligence Scale for children 2Statistical tests were used to compare characteristics of participants who had IQ measured at 8 or 15 years, against participants who had no IQ measurements. Continuous variables were compared by using independent t-test and categorical variables were compared by using χ2 tests. 3Maternal education is reported as the highest completed level on five ordinal categories from Certificate of Secondary Education(CSE), Vocational training, O(ordinary)-level (taken by the top 25% of CSE at 15 years), A(advanced)-level (involving 2 years of study beyond O-level) and degree or higher. The CSE, O-levels and A-levels are completed at secondary school. 4Social class was categorized according to maternal occupation during pregnancy, according to standard UK classifications of occupation, ranging from class I (highest), II, III-non-manual, III-manual, IV, and V (lowest)[17]. 5Stimulation in the home environment was measured by an adaptation of the HOME questionnaire at 18 months of age[18].