IQ stands for "intelligence quotient." The IQ test measures intellectual and spatial abilities such as those used for memory, language and mathematics. A person's IQ used to be calculated from the test results as a ratio of mental age to chronological age, multiplied by 100. Now the values of the IQ are mostly based on a bell-shaped curve, where mean intelligence, designated as 100, is at the top of the curve. A majority of people with average intelligence will have an IQ of 100 plus or minus 15, the standard or error range. Among the available IQ tests, the Stanford-Binet and Wechsler tests are used most often. Both tests have evolved over the years and sub-tests can be added to address specific goals.
Choose the right test to achieve your goal. Tests that target specific needs or sub-populations are also called "scales." The Stanford-Binet scales are good at differentiating the test results of values that fall at the ends of the bell-shaped curve such as for mentally challenged children and highly gifted individuals. The Wechsler IQ test has two main scales, one for children and one for adults. There is also a Wechsler Preschool and Primary Intelligence scale, which tests children who do not yet read or write. In general, the Stanford-Binet test emphasizes verbal abilities, while the Wechsler IQ test incorporates more non-verbal abilities such as visual-spatial abilities.
Obtain your raw score. An IQ test has many parts. When you complete the test, all subtest scores are added and normalized to the bell-shaped curve, where the mean score for the test is equal to 100.
Obtain your normalized score. The bell-shaped curve or "normal distribution" was standardized by testing large numbers of test takers. A normalized score of a 100 means that 50 percent of people scored lower and 50 percent scored higher. Taking the test at different times and under different conditions can introduce variations. Therefore, the standard deviation of plus or minus 15 indicates a score in the range of 85 to 115, which places you in the average intelligence category.
Interpret your score. Standardized IQ tests are very good at predicting future academic ability and achievement of children, but the tests don't necessarily predict future success in a career or life. Interpretation of scores for children should be done in consultation with a trained professional or psychologist, especially when placement decisions have to be made. A score, signifying normal intelligence, is no more than one standard deviation or 15 points away from 100 and so falls in the 85 to 115 range. A low or high score is two standard deviations away from 100. The low score would be in the 70 to 85 range and the high score would fall between 115 and 130. A score less than 70 is considered very low and usually is associated with serious disability, while a score above 130 is considered superior intelligence or genius.