Our mental age can be above or below our age mates, up to age 16. For adults, we can express the comparison to our age peers as a percentile. To be valid a test should correlate with another, relevant measure e.g. school performance or job performance. Some argue some intellectual tests are biased towards some cultures. Certainly, subtests of a test should not favour one group or another without a clear justification.
Intelligence tests are useful for diagnosing specific learning disabilities, it is thought.
Mental retardation is often indicated by a low IQ score on a test that incorporates a wide range of abilities. Mild mental retardation is an IQ score from 52- 67, MA 7 to 11. Mildly retarded people can hold jobs, and be independent. Below IQ 20 likely there will be noticeable lacks in physical and social skills. Many are interested in the roles of heredity and environment in setting mean scores for groups of individuals,such as families, and variations between groups such as between families or cultural groups.