The General Intelligence Factor (g)
There are many different kinds of IQ tests using a wide variety of methods. Some tests are visual, some are verbal, some tests only use of abstract-reasoning problems, and some tests concentrate on arithmetic, spatial imagery, reading, vocabulary, memory or general knowledge.
The psychologist Charles Spearman early this century made the first formal factor analysis of correlations between the tests. He found that a single common factor explained for the positive correlations among tests. This is an argument still accepted in principle by many psychometricians. Spearman named it g for "general intelligence factor." In any collections of IQ tests, by definition the test that best measures g is the one that has the highest correlations with all the others. Most of these g-loaded tests typically involve some form of abstract reasoning. Therefore Spearman and others have regarded g as the perhaps genetically determined real essence of intelligence. This is still a common but not proven view. Other factor analyses of the data with different results are possible. Some psychometricians regard g as a statistical artifact. The accepted best measure of g is Raven's Progressive Matrices which is a test of visual reasoning.