© 2004 Paul Cooijmans
As written elsewhere, I am opposed to expressing I.Q.s in relation to age peers (or whatever limited groups) and regard adult deviation scores as the true I.Q.s. This implies that in my view scores of children must be expressed by adult norms. As they rarely are, I describe below a rough method to convert childhood age peer scores to adult scores. Note this method must not be used in cases where the score has already been expressed by adult norms!
Take the age peer I.Q. and the child's age when the test was taken. Calculate the child's "mental age", as if the I.Q. were a mental/biological age ratio score as in the past.
Now convert the mental age to a mental/biological age ratio score of a hypothetical person of age 16. This result is an approximation of the child's adult I.Q. at the moment the test was taken.
A 10-year old scores 170.
Calculate mental age: 1.70 times 10 equals 17.
Convert to ratio score of hypothetical person aged 16: 17 divided by 16 times 100 equals 106.
So this 10-year old has an adult I.Q. of 106.
A 5-year old scores 275.
2.75 times 5 equals 13.75
13.75 divided by 16 times 100 equals 86.
So this 5-year old has an adult I.Q. of 86.
An 11.5-year old scores 180.
1.8 times 11.5 equals 20.7.
20.7 divided by 16 times 100 equals 129.
So this 11.5-year old has an adult I.Q. of 129.
When I.Q. is seen this way, it must be understood as a property that increases during childhood and reaches its peak in adulthood. The steep increase of intelligence typical of childhood ends at about age 16. The further increase thereafter is shallow.