M: mean; SD: standard deviation. The theoretical ranges for each measure with a defined upper limit are as follows: reasoning 0–20, memory 0–30, and semantic knowledge 0–54. The cognitive activity scores are on a normal metric with means of approximately 0 and SD of approximately 0.8.
aThe 1986 cohort was followed for up to 18 years and the 1993 cohort for up to 12.
bThe reasoning measure was not given until year 6 for the 1986 cohort.
cThe memory measure was not given in year 18.