We explain IQ scale, average IQ and ranges

IQ stands for intelligence quotient. It all started with french psychologist Binet. Binet wanted to to calculate whether a child was over or under the average mental development of children of the same age.

To do that, he developed mental tests that were given to children of different years. Then, he calculated the average score for every age. After some hard-work he obtained the IQ averages for each age.

Now when a new child was tested, he would check the performance against children of the same year. If a boy was 14 years old and scored 25 in the test, while the average for his age was scoring 22, Binet concluded that it was more intelligent than the average. He looked into which age had that average, for example let us imagine that a children of 15 years old called Tom scored an average of 25, then he would catalogue the kid as mentally aged 15 years old.

A critical story twist comes from german psychologist W. Stern, who introduced chronological each in order to obtain a good and easily comparable number. IQ as a concept was born. To calculate it, he would divide scored age (which age had for average the score achieved by the kid) by real age. In Tom's example, 15 by 14, which is 1,07.

Since working with decimals was inconvenient, he introduced a multiplication by 100, to avoid them. So Tom obtained 107 of IQ for someone of his age.

However, when psychologists started using the IQ test for adults they soon understood that age was not relevant so they needed another way of comparing intelligence between adults. To do that, they simply compared to a sample of other adults, they painted them in a bell curve and used the IQ Scale, let's us understand it in the next section.