In order to get a full and true measure of the correlation between two tests, the following precautions are necessary: (1) The _same individuals_ must be given both tests. (2) The number of individuals tested must be as great as 15 or 20, preferably more. (3) The individuals should be a fair sample of the population in regard to the abilities tested; they should not be so selected as to represent only a small part of the total range of ability. (4) The tests should be thorough enough to determine each individual's rank in each test, with a high degree of certainty. Sloppy testing gives a correlation nearer zero than it should be, because it "pies" the true orders to some extent. [End footnote] {285} General Factors in Intelligence If now we try to analyze intelligence and see in what it consists, we can best proceed by reviewing the intelligence tests, and asking how it is that an individual succeeds in them. Passing the tests is a very specific instance of {286} intelligent behavior, and an analysis of the content of the tests should throw some light on the nature of intelligence. The first thing that strikes the eye in looking over the tests is that they call for so many different reactions. They call on you to name objects, to copy a square, to tell whether a given statement is true or false, to tell wherein two objects are alike or different. The first impression, then, is that intelligence consists simply in doing a miscellaneous lot of things and doing them right. But can we not state in more general terms how the individual who scores high in the tests differs from one who scores low? If you survey the test questions carefully, you begin to see that the person who passes them must possess certain general characteristics, and that lack of these characteristics will lead to a low score. We may speak of these characteristics as "general factors" in intelligent behavior. First, the tests evidently require the use of past experience. They call, not for instinctive reactions, but for previously learned reactions. Though the Binet tests attempt to steer clear of specific school knowledge, they do depend upon knowledge and skill picked up by the child in the course of his ordinary experience. They depend on the ability to learn and remember. One general factor in intelligence is therefore _retentiveness_.