Encyclopedia of Psychology Alan E. Kazdin, PhD, Editor-in-Chief
INTELLIGENCE.
...The choice of the problems to be included in any measure of intelligence is not a critical issue. It is now well established that people who excel in one type of intellectual task are likely to excel in other intellectual tasks.
...There are thousands of kinds of tasks that are positively related to one another that may be presented to people such that people who excel in one type of task are likely to excel in other unrelated intellectual tasks.
...The index derived from such a test is called a measure of general intelligence, or g. This term, introduced by Charles Edward Spearman in 1904, may be defined as the common element present in a diverse set of intellectual measures that are positively related to each other.
...Intelligence quotients, or IQ test scores, are measures of g based on comparisons of the performance of individuals of the same age who take the same test. It is conventional to assign the value of 100 to the average score on the test. The tests are also scored in such a way that approximately 68% of the people given the test score between 85 and 115.