Question: how is iq calculated? 

Answer: IQ is a normalized statistic with an arbitrarily set mean of 100 and a standard deviation of 15.\n\nWhat does that mean?  It means that the "average" person has an IQ of 100.  People with an IQ over 100, then, are among the "smart half" of the population, while people with an IQ below 100 are among the "not-so-smart half."  \n\nSince the standard deviation is set at 15, we can use IQ as a comparison tool.  For example, a score below 70 (which only occurs among about 2.5% of the population) is often used to indicate mental retardation.  Similarly, a score above 130 (also occuring among 2.5% of the population) indicates intellectual giftedness.  Most people (about 68%) have an IQ between 85 and 115.\n\nHow accurate and useful is all of this?  Keep in mind that an IQ test is only one way (and a limited one at that) of measuring the intelligence of an individual.  People who are "smart" in linguistic and mathematical areas will perform better on most IQ tests.  That does not account for people who are gifted in other ways - star athletes, musicians, artists, people who have a knack for getting along well with others, etc.
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