1. Which of the various tests correlate with each other?
2. What correlation exists between mental and motor abilities?
3. Do the tests measure fundamental qualities or general powers of the individual, or specialized capacities, or perhaps mainly the effect of general or special training?
4. If they measure general qualities, which of the existing tests are the best for this purpose?
5. How many trials are needed to afford a reliable index of the individual's ability?
6. What are the principal incidental factors that influence the result of tests?
7. Which tests are most easily influenced or disturbed by extraneous factors?
8. Can tests of the simpler laboratory type be used to indicate the individual's ability as shown in his daily work and play?
9. How simple or complex should the various tests be in order to give the best results?
10. How many tests, and which, are required to give a fairly correct picture of the individual's psychological make-up?
11. To what degree do preliminary trials indicate the final capacity of an individual?
12. Does the intercorrelation of tests change in any way with practice, repetition, and familiarity with the material?
13. Just what mental functions may the particular tests be said to measure?
14. How important are these functions in practical, educational and vocational life?
15. By what amounts and in what various ways do individuals differ among themselves in such abilities as the tests measure?
16. Are there other important aspects of psychological constitution and equipment for which there now exist no adequate tests?
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.