Intelligence Quotient, or IQ, is measured through a standardized test called an IQ test, which gives an individual a standardized score that can be compared to the population as a whole. IQ scores usually follow a normal distribution such that most scores are near the average.
As of 2014, the Wechsler Adult Intelligence Scale—Fourth Edition is the IQ test most often administered to adults; a comparable version designed for children is administered to people 16 years of age and under. On this test, 68 percent of the scores fall between 85 and 115, with the average being 100. Only about 0.2 percent of the population receives scores below 55, which demonstrate a low IQ, or scores above 145, which demonstrate a high IQ. The interpretation of a score is determined by the range it falls in. For example, a person with an IQ score of between 145 and 159 is considered highly gifted while a person with an IQ score of between 55 and 69 is considered to have a mild mental disability.
IQ tests measure both crystallized and fluid intelligence. Crystallized intelligence refers to the skills and knowledge one has acquired through life, while fluid intelligence refers to a person's problem-solving and reasoning abilities.