The next step in the development of tests consisted in the cooperative effort to standardize the nature and methods, the conditions and mode of record. Many hands had part in this process, until in recent years, through publication, comparison and discussion of the subject, fairly uniform principles of technique, record, and treatment of measures have been agreed upon. This made possible the comparison of results secured by different investigators, and facilitated the statistical treatment of the data, so that later work might profit by what had already been tried or accomplished by earlier workers. After many years of this sort of cooperative work, another series of studies was inaugurated to attempt what has come to be known as "testing the tests." These studies proceeded by examining into the degree to which the various tests correlate with each other, with other indications of the individual's ability, with age, sex, health, education, school standing, special training, etc. Such questions as the following will suggest the problems involved in "testing the tests." 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?