He hypothesized that there should exist a correlation between intelligence and other observable traits such as reflexes, muscle grip, and head size. A pioneer of psychometrics and the application of statistical methods to the study of human diversity and the study of inheritance of human traits, he believed that intelligence was largely a product of heredity (by which he did not mean genes, although he did develop several pre-Mendelian theories of particulate inheritance). The English statistician Francis Galton (1822–1911) made the first attempt at creating a standardized test for rating a person's intelligence. Both intelligence classification by observation of behavior outside the testing room and classification by IQ testing depend on the definition of 'intelligence' used in a particular case and on the reliability and error of estimation in the classification procedure. Those other forms of behavioral observation are still important for validating classifications based primarily on IQ test scores. Historically, even before IQ tests were devised, there were attempts to classify people into intelligence categories by observing their behavior in daily life. See also: History of the race and intelligence controversy Precursors to IQ testing Investigation of different patterns of increases in subtest scores can also inform current research on human intelligence. Raw scores on IQ tests for many populations have been rising at an average rate that scales to three IQ points per decade since the early 20th century, a phenomenon called the Flynn effect. They are also used to study distributions of psychometric intelligence in populations and the correlations between it and other variables. In research contexts, they have been studied as predictors of job performance and income. IQ scores are used for educational placement, assessment of intellectual disability, and evaluating job applicants. While the heritability of IQ has been investigated for nearly a century, there is still debate about the significance of heritability estimates and the mechanisms of inheritance. IQ scores have been shown to be associated with such factors as nutrition, parental socioeconomic status, morbidity and mortality, parental social status, and perinatal environment. Unlike, for example, distance and mass, a concrete measure of intelligence cannot be achieved given the abstract nature of the concept of 'intelligence'. Scores from intelligence tests are estimates of intelligence. This results in approximately two-thirds of the population scoring between IQ 85 and IQ 115 and about 2 percent each above 130 and below 70.

For modern IQ tests, the raw score is transformed to a normal distribution with mean 100 and standard deviation 15. The resulting fraction ( quotient) was multiplied by 100 to obtain the IQ score. Historically, IQ was a score obtained by dividing a person's mental age score, obtained by administering an intelligence test, by the person's chronological age, both expressed in terms of years and months. The abbreviation 'IQ' was coined by the psychologist William Stern for the German term Intelligenzquotient, his term for a scoring method for intelligence tests at University of Breslau he advocated in a 1912 book. An intelligence quotient ( IQ) is a total score derived from a set of standardised tests or subtests designed to assess human intelligence.

Best online iq test