Simply stated, your Intelligence Quotient (IQ) is designed to measure your ability to learn. Therefore, an IQ test is not meant to test what you know, but rather your potential for knowing. In other words, smart is not the same as knowledgeable.
Early Tests
The first attempt at the modern IQ test was the Binet test by Alfred Binet and Theodor Simon. This test was created to allow educational institutions to identify children who might need special education because of retarded mental development. It involved such things as asking a child to compare the weights of objects, or observing as they unwrapped a piece of candy. To anyone interested in research methodology, these tests are almost laughable, but these are the humble beginnings of our attempt to understand the workings of the human mind.
Age Accountability
The test slowly evolved to account for age and its relations to mental development. It followed the formula:
IQ = (Mental Age / Chronological Age) * 100)
For example, if a certain child could do basic multiplication at the age of 4 (CA) when the average child cannot grasp the concept until age 8 (MA), that child would have an IQ of 200. Obviously, this is a gross simplification of a complex system, though it explains the basic idea.
Deviation Accountability
These tests were quite useful for measuring the intelligence quotient for children, but failed when it came to measuring adults. This is because, measurable tests of ability started to plateau between the ages of 16 - 20. After this, dividing by the chronological age simply resulted in lower and lower scores for adults. Because of this, the deviation based tests were used. These tests compared the scores for groups of people in the same age group. This system assumes that IQs will follow a normal distribution (i.e., the standard bell curve), and that the mean score is 100 with the standard deviation being approximately 15.
Scores and Their Meanings
Classifications under the Stanford-Binet test
One of the original systems for classifying people by their IQ was created by Lewis M. Terman in his book The Measurement of Intelligence. In it, he defines these generalities:
Classification IQ Score
-------------------------------------
Genius/Near Genius 140+
Very Superior 120-140
Superior 110-120
Normal or Average 90-100
Dullness 80- 90
Borderline Deficiency 70- 80
Definite Feeble-Mindedness below 70
At this time, anything below "Dullness" was further categorized. These classifications were changed because they were abused by the public (Moron! You are such an idiot!), and are now obsolete when referring to IQ.
Classification IQ Score
-------------------------------------
Moron 50- 69
Imbecile 20- 49
Idiot below 20
Wechsler Adult Intelligence Scale (WAIS)
The more modern system is the WAIS. Partially because of the negative connotations society put on the original classifications (i.e., they became insults rather than measurements), the new system was far more general in its naming scheme. It still, however, closely resembled the original:
Classification IQ Score
-------------------------------------
Very Superior 130+
Superior 120-129
High Average 110-119
Average 90-109
Low Average 80- 89
Borderline 70- 79
Extremely Low below 70
As with the old system, the WAIS further categorizes anything below borderline. Today, all of these are grouped together and are typically referred to as mental retardation, though even that is not considered PC anymore.
Classification IQ Score
-------------------------------------
Mild 50- 69
Moderate 35- 49
Severe 20- 34
Profound below 20