IQ stands for intelligence quotient, a label assigned to the score achieved on an IQ test. Originally, the IQ test was intended to accurately compare a person's chronological age with his mental age and express it as a ratio: 100 times the mental age divided by the chronological age. As an example, a 10-year-old boy with the thinking and reasoning ability typical of a 15-year-old would have an IQ of 100 multiplied by the result of 15 divided by 10, or 150. Scores range from 0 to 200, with 100 representing the average, where mental age equals chronological age. However, many prefer test results expressed in terms of percentile, comparing one person's score with that of the rest of the population's -- which gives the numbers context. For example, Mensa, a society for people with high IQ levels, typically accepts as members those who score in the top 2 percent of the population, regardless of the actual scores this percentage encompasses.
In 1904, French psychologist Alfred Binet devised the first IQ test, in which results were expressed as a mental age. The test never caught on, however. The revised version, called the Stanford-Binet test, expresses results as a score instead of a mental age but is appropriate only for testing children, not adults. The fifth edition of this test offers not just one score but several: abstract and visual, verbal reasoning, a quantitative score measuring math skills and a short-term memory score.
Unlike the Stanford-Binet, the Wechsler test is available in two age-appropriate versions: the Wechsler Intelligence Scale for Children and the Wechsler Intelligence Scale for Adults. The tests have been revised several times. The January 2009 versions express results as percentiles and deviations, or ranges in comparison to population averages. Divided into two main categories -- verbal and performance -- the test is made up of several sub-tests addressing knowledge of factual information, similarities or ability to categorize, arithmetic, vocabulary, comprehension, digit span or short-term auditory memory, picture completion, arrangement of blocks to match a design, figure weights, visual puzzles and cancellation. The adult test also addresses matrix completion, letter-number sequencing and symbol search.
Some completely nonverbal tests -- convenient for non-English speakers -- include the Leiter test, which is appropriate up to 27 years of age; Raven Advanced Progressive Matrices, and the Test of Nonverbal Intelligence. Others include the Slossen tests, the California Test of Mental Maturity and the Cattell Culture-Fair Test. Untimed, unproctored tests such as the Mega Test, the Test for Genius and the Titan Test are considered less reliable, because they're vulnerable to cheating and results are determined by factors other than intelligence -- such as persistence and library skills.
Some criticize that the tests don't actually measure intelligence and how the brain works because they don't cover all areas in which a test-taker might be extraordinarily talented. Also, some people are especially gifted in some areas but lacking in others; the total score does not always reflect the weakness and the gift but averages them to a mediocre number.