children [Bin11]. It was found that Binet's test results were a good predictor of children's academic performance.

Lewis Terman of Stanford University developed an English version of Binet's test.

As the age norms for French children did not correspond well with American children, he revised Binet's test in various ways, in particular he increased the upper age limit.

This resulted in the now famous Stanford-Binet test [TM50]. This test formed the basis of a number of other intelligence tests, such as the Army Alpha and Army Beta tests which were used to classify recruits. Since its development, the Stanford-Binet has been periodically revised, with updated versions being widely used today.

David Wechsler believed that the original Binet tests were too focused on verbal skills and thus disadvantaged certain otherwise intelligent individuals, for example the deaf or people who did not speak the test language as a first language. To address this problem, he proposed that tests should contain a combination of both verbal and nonverbal problems. He also believed that in addition to an overall IQ score, a profile should be produced showing the performance of the individual in the various areas tested. Borrowing significantly from the Stanford-Binet, the US army Alpha test, and others, he developed a range of tests targeting specific age groups from preschoolers up to adults [Wec58]. Due in part to problems with revisions of the Stanford-Binet test in the 1960's and 1970's, Wechsler's tests became the standard. They continue to be well respected and widely used.

Owing to a common lineage, modern versions of the Wechsler and the Stanford-Binet have a similar basic structure [Kau00]. Both test the individual in a number of verbal and non-verbal ways. In the case of a Stanford-Binet the test is broken up into 5 key areas: Fluid reasoning, knowledge, quantitative reasoning, visual-spatial processing, and working memory. In the case of the Wechsler Adult Intelligence Scale (WAIS-III), the verbal tests include areas such as such as knowledge, basic arithmetic, comprehension, vocabulary, and short term memory. Non-verbal tests include picture completion, spatial perception, problem solving, symbol search and object assembly.

As part of an effort to make intelligence tests more culture neutral John Raven developed the progressive matrices test [Rav00]. In this test each problem consists of a short sequence of basic shapes. For example, a circle in a box, then a circle with a cross in the middle followed by a circle with a triangle inside. The test subject then has to select from a second list the image that best continues the pattern. Simple problems have simple patterns, while difficult problems have more subtle and complex patterns. In each case however, the simplest pattern that can explain the observed sequence is the one that correctly predicts its continuation. Thus, not only is the ability to recognise patterns tested, but also the ability to evaluate the complexity of different explanations and then correctly apply the philosophical principle of Occam's razor. We will return to Occam's razor and its importance in intelligence testing in Subsection 3.3 when considering machine intelligence.

Today several different versions of the Raven test exist designed for different age groups and ability levels. As the tests depend strongly on the ability to identify abstract patterns, rather than knowledge, they are considered to be some of the most "g-loaded" intelligence tests available (see Subsection 2.5). The Raven tests remain in common use today, particularly when it is thought that culture or language bias could be an issue.

The intelligence quotient, or IQ, was originally introduced by Stern [Ste12]. It was computed by taking the age of a child as estimated by their performance in the intelligence