The investigation of these numerous problems has resulted in the accumulation of a considerable literature of mental tests. Many of the earlier forms of tests were abandoned because of their unsatisfactory or meaningless character. Others have been retained and improved in form, and many new ones are constantly being devised and elaborated, described and standardized. The precautions to be observed, the instructions to be given, and the methods of record and interpretation have been presented in various books and manuals. The tests have been developed for more and more complex functions, and now relate not only to relatively simple capacities but to highly elaborate and subtle forms of achievement. As rapidly as is consistent with accuracy, norms and standards of performance for different ages, school grades, vocational requirements, etc., are being accumulated and reported. Typical charts of age norms in selected tests are given in the Appendix. As the tests have thus developed they have been organized for a variety of special purposes, such as for school measurement, educational diagnosis, clinical examination, laboratory experiment, and more recently for the purposes of vocational guidance and selection. Among the first of these to develop systematically, and also the ones with the most immediate vocational application, are the graded intelligence scales, which shall be our next concern. GRADED INTELLIGENCE SCALES AND NORMS An important step in the history of general tests is represented by the accumulation of norms and standards of performance for the different selected tests, and the arrangement of scales of tests with increasing difficulty, as further aids in fixing the individual's status. After a standardized and tested form of test has been selected, norms of performance are accumulated by applying the test to large numbers of persons of the same general type. The classification may be on the basis of age, school grade, occupation, nationality, etc. In this way it becomes possible to determine for a given individual how he compares with other members of his group; whether he is above or below the average, and how far; whether he would belong among the best ten, or the poorest ten, or the third ten, etc., of one hundred selected at random. Such norms also reveal to what degree the tested ability varies with the other factors, on the basis of which the group was selected, as age, sex, education, size, health, race, etc.