A quantitative representation of cognitive ability which results from testing a sample of cognitive skills. The formula is intellectual age divided by chronological age, times 100. For example, someone 10 years old with an intellectual age of 13 would have an IQ of 130. This is called the "ratio IQ."
The scales of different IQ tests vary slightly due to differences in test construction and the sample which provided the norm. Variation in scores is described by the standard deviation. Assuming that intelligence is normally distributed, the IQs of about 95 percent of the population are between 70 (about 2 standard deviations below the mean) and 130 (about 2 standard deviations above the mean). Below 70 is considered retarded, and above 130 is considered gifted. Individual tests such as the WISC and Stanford-Binet are considered the most reliable, but no published test since the older Stanford-Binet Form LM (1972) is valid above 160. Most IQ tests since 1960 have reported IQ as "deviation IQ," which adjusts the ratio IQ scale slightly based on the different means and standard deviations of each age group in the sample used to construct the test. Ratio and deviation IQ's seldom differ by more than 4 points. See levels of giftedness, ceiling effect, multiple intelligences.

This definition is in context to Gifted Education. See more contextual defintions for Intelligence Quotient (Iq).