Tests of the abilities of human beings may be classified upon a great many different bases. It is possible, first of all, to classify them according to the qualities of mind and body which they measure. The reason it is difficult so to classify tests of mental ability is that the mind refuses to be cut up into different parts, each one responsible for a specific characteristic. No test can be solved by the use of one and only one group of intellectual faculties. The results obtained in any mental examination are the complex effects of an immense number of different characteristics. No attempt has therefore been made in the classification of the Mentimeters to say that one measures imagination, another measures attention, and another some other quality. Almost every quality enters to some degree in each test. It is possible to classify tests according to the subject matter which they contain. The Mentimeter tests are so arranged, where it is possible, as to cover a very wide range of subject matter. It is possible to classify examinations according to the activity required of the candidate being examined. A number of the Mentimeter tests call for _completing_ a series of objects or ideas, while a number of others call for _memory_ of a certain sort, and still others require _discrimination_ between certain differing elements. These differences in the activity of the candidate examined, are not, however, the chief distinctions to be made between the tests. It is possible to classify measurements according to the number of candidates that may be examined at the same time. Some tests cannot be given readily to more than one person at a time, while other tests can be given to several at the same sitting. In so far as possible, the Mentimeter tests are so arranged that they can be given to large numbers at the same sitting. This makes for economy of time and of effort on the part of the examiner. It is possible to classify tests according to physical characteristics of the candidate examined, such as tests for infants, tests for children, and tests for adults, or tests for the blind and tests for the deaf. The first test in the Mentimeter series is for infants while the remainder of the tests are intended to measure older people. Tests may further be classified according to the language capacity of the candidates who are examined. Certain of the Mentimeter tests are for non-English-speaking persons primarily, while others are primarily for those who speak English, and still others for those who read English.