Human Growth and Development / Unit 7 HGD 0937 short lesson, about 20 seconds
Ratio IQ and Deviation IQ
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The idea
Ratio IQ, the historical formula, is mental age divided by chronological age, times one hundred; modern tests use deviation IQ, a score's distance from one's age-group norm.
In practice
A ten-year-old performing like a twelve-year-old scoring one hundred twenty on the ratio formula.
Where people slip
The misconception is that modern IQ is still mental age over real age; deviation scores replaced it.
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