It must not be thought that the result of any test is always 100 per cent. accurate. Even good workmen sometimes make errors in the use of the most precise scientific instruments. Even though constructed with the most painstaking care, according to the truest scientific formulas and by men of the highest technical training and skill, the mechanical instruments of precision are occasionally found to be inaccurate. If this is the case with material implements and dimensions which are finite, concrete, and tangible, how much greater is the liability to error in dealing with the intangible, infinite, and more or less abstract qualities of the human mind. The scientific psychologist is, after all, merely another human being, and as such equally liable with all other human beings to human error. Of no man or woman can it be said that he or she is infallible, and as every one who applies a psychological test is human, and so liable to error either in its application or the reading of its result, conclusions drawn from the results of any particular test should be accepted as accurate only when they have been checked by the results of other tests applied to the same subject, and substantial conformity of the results of one to those of the others has been obtained. For this reason, among others, no single test can be expected to yield definite and complete information as to any particular individual’s mental capacity or ability, whether gauged by the universal standard or by group comparison. It has, therefore, been necessary to establish, as preliminary to the preparation of the Mentimeter tests, a variety of standards, and to prepare a considerable number of tests under each of these standards, all or most of which must be used in each instance if anything approaching scientific accuracy is to be reflected in the resulting scores. As has previously been pointed out, however, the scientific method is incomparably freer from the liability to error than any method of determining human ability and capacity that depends upon unaided personal observation. How completely this has been demonstrated in practice in a wide range of fields is set forth in subsequent chapters. To yield results of maximum accuracy, however, scientific mental tests must be used only with reference to the standards on which they are based. Lest it has not been made clear already to the reader how the method of establishing mental standards of comparison operates, let us again briefly try to point out just what is meant by a universal standard of mental capacity.