【Definition】

What is intelligence? The definition of intelligence is "the ability to learn or understand or to deal with new or trying situations” or “the skilled use of reason". Some people misuse the word smart to mean knowledgeable. That is like confusing velocity with distance. That one can lead to the other does not mean that they are the same thing.

I.Q. = Intelligence Quotient

To relate the mental development of a child to the child's chronological age the IQ was invented. IQ = (MA/CA) * 100. The intelligence quotient was equal to 100 times the Mental Age divided by the Chronological Age. For example, if a certain child started reading, etc., at the age of 3 (CA) and average children start reading, etc., at the age of 6 (MA), the child would get an IQ score of 200. (Such a score is very, very rare). Since people wanted to also use IQs for adults, that formula was not very useful since raw scores start to level off around the age of 16 (2).

Thus the deviation IQ replaced the ratio IQ. It compares people of the same age or age category and assumes that IQ is normally distributed, that the average (mean) is 100 and that the standard deviation is something like 15 (IQ tests sometimes differ in their standard deviations).

What is a standard deviation (SD)? Simply put, the standard deviation is a measure of the spread of the sample from the mean. As a rule of thumb, about 2/3 of a sample is within 1 standard deviation from the mean. About 95% of the sample will be within 2 standard deviations from the mean (3).

With the standard deviation and a mean, you can calculate percentiles. Percentiles tell you the percent of people that have a score equal to or lower than a certain score.

A typical classification system is as follows: