Once, however, tests have been applied to the supposititious thousand expert engineers, and the performance of each of them in each test has been given its proper place in the scale, and an average struck, there has come into existence a preliminary standard; which, however, before being offered for general use in the testing of engineering students and others, must first be tried out by experimental application on as many individuals and groups as are available, and their performance with reference to the standard checked up by all other means available. It may be, and quite frequently is, the case that this preliminary try-out of a standard results in the elimination of some of its elements, the modification of others, and the necessary preparation of a new series of tests based upon the altered standards. But in this fashion, in the course of time and as the result of the combined effort of many trained minds, there is at last set up a standard which is substantially universal in its application, and by which it may readily be determined whether or not any particular individual possesses the mental capacity and particular abilities that have been found to be necessary if he is to develop into a competent engineer. As psychological tests are more and more widely applied and there is consequently accumulated an increasing volume of data which can be collected, classified, and compared, standards become either more firmly established as a result of experience or subject to modification in the light of the wider range of knowledge. In science nothing is final. What psychology offers to-day is a method of mental tests, the soundness of which in principle is unchallenged, though the application in detail of these principles is subject to constant improvement and refinement. CHAPTER V DIFFERENT TYPES OF MENTAL TESTS The character of any mental test or series of tests is determined primarily, of course, by the purpose for which the test is applied, and, secondarily, by the known or obvious mental limitations of the individual under examination. Mental tests thus classify themselves, in the first instance, into as many different classes as there are specific purposes to be served by their use, particular kinds or classes of mental ability and capacity to be ascertained, or degrees of previously known mental limitations. Each one of these classifications cuts across all other classifications at some point, so that it is, as a matter of practice, impossible to tabulate or catalogue mental tests in such a way as to separate them into sharply defined or permanently detached groups or classes.