Broadly, all mental tests subdivide at first into tests devised for use with persons of normal mental capacity and development and tests for intelligences that are not fully developed. This is, perhaps, the chief permanent and fixed classification of intelligence tests that can be made, for in a group of tests for the sub-normal mind would be included the entire series of tests adapted for the examination of the mental powers of children of all ages, from earliest infancy to maturity. In fact, the standard method of rating or grading adults of undeveloped or sub-normal intelligence is to classify them by their mental age as compared with the performance of normal children of the same age. Thus, a man or woman of twenty-five who is able to make a high score in tests which are passed successfully by normal children of eight, but who fails when subjected to tests which a normal child of ten should pass easily, is rated approximately as of mental age nine. Cutting across this classification is the arbitrary classification of tests adopted in the psychological work of the United States Army, in which every officer and enlisted man is classified as to his relative intelligence by means of scientific mental tests. The Army tests are of three principal kinds. There is a series of tests, known as the Alpha, designed to measure the intelligence of individuals who can read and write the English language. For those who are either illiterate or whose ability to read or write is confined to some language other than English, there is the Beta series of tests. These may register as high a degree of intelligence as the Alpha tests; the results are merely not expressed in terms of the English language. The third classification in the Army is the individual tests, applied to those who fail to make a satisfactory score under either the Alpha or the Beta tests. This is, in its Army application, a system of tests for the sub-normal adult intelligence. Thus the broad classification first set forth above, in substance actually holds in the classification of the Army tests. Under each of these two broad classifications, and particularly under the first (since in general, every-day practice it is of little service to undertake to analyze minutely the capacities and limitations of the sub-normal mind except in the application of these tests to growing children) there are many possible subdivisions of mental tests, based upon the particular mental qualities which it is desired to measure.