One common question that people have about intelligence tests is how IQ is calculated. This is an important question because the most important result of an intelligence test is the IQ. Having a transparent process for calculating the score is important for building trust.
In the Olden Days . . .
The original equation for calculating IQ was created by German psychologist Wilhelm Stern over 100 years ago:
In this equation, the “chronological age” was the examinee’s actual age (in years). The “mental age” was the age of the average person who performed as well on the test as the examinee. If, for example, a 10-year-old performed as well as the average 9-year-old on the test, then the child’s IQ would be 9 / 10 x 100 = 90.
Because the most important part of the equation uses division, Stern called this score the “intelligence quotient,” or IQ. Today, we call this score a “quotient IQ” to distinguish it from the IQs calculated with modern methods (more on that later).
Problems with Stern’s IQ
Stern’s IQ equation was quickly adopted by intelligence test creators, and it persisted for decades. However, it had problems. One was that IQs were generally not comparable across ages. For example, if a child was two years ahead of her peers’ mental development, she would have an IQ of 140 at age 5 (because 7 / 5 x 100 = 140) and an IQ of 125 at age 8 (because 10 / 8 x 100 = 125). Even though she is still two years ahead of her classmates, her IQ decreased from age 5 to age 8!
Stern’s IQ also did not apply well to adults. After all, we expect children’s cognitive abilities to improve with age. We don’t really expect that from adults. Most psychologists accommodated this by standardizing the “chronological age” to 16 or 18 for all adults, but this was unsatisfactory. There is strong evidence that people’s mental abilities continue to increase past that age--especially if they attend school.
