Intelligence tests are instruments used in psychology to measure cognitive ability. The intelligence quotient (IQ) is an index that summarizes the performance of a given individual on all subtests of an intelligence test. The IQ score is taken as an indicator for general intelligence or g. There are various theories about what g is. In addition, much effort has been devoted to establishing group differences in g. However, intelligence test scores can only be meaningfully compared when measurement bias is absent, which is, for instance, not the case in research into the increase of IQ over time (the Flynn effect) and into differences in national IQ.