It is easy enough to determine that a delicate, small-boned, slender person is not the best type to employ for digging coal, loading freight cars, or other arduous manual labour. There are, of course, many classes of occupations the fitness or unfitness for which of a particular individual must be determined in the first place by that individual’s physical characteristics. So far the observation method suffices. But the very fact that every industry and business is full of misfits and that it is a matter of common knowledge that the most difficult problem the employer has to face is that of finding the right person for each particular job that calls for anything more than mere physical strength, is the best evidence that even the most experienced and accurate observers are far from infallible in their judgments of individual capacities. For that matter, there is no infallible test. No true scientist claims infallibility. The possibility of error is always present wherever the human element is involved. It is a safe assumption that any method or estimate that purports to be infallible is fraudulent. There is in almost every human mind a lurking, subconscious belief in the possibility of perfection. It is this which makes humanity credulous when claims of infallibility are plausibly presented. It is extremely difficult to satisfy by logic and reason the type of mind that is strongly influenced by glittering generalities and emphatic, though unsupported, assertions. It is equally difficult to convince the skeptic whose mind is closed to the introduction of new thoughts and who, in his self-satisfaction with his own mental limitations, rejects every fact that does not tally with his preconceived ideas. This book is written neither for the super-skeptical nor the ultra-credulous. It makes no pretension to infallibility, nor does any scientifically trained psychologist pretend that there has yet been evolved a method of measuring every dimension and capacity of the human mind beyond the possibility of error. The methods described in this book are the fruit of years of experiment, research, and practical application of the results of experiment and research, and are designed to reflect the development of the science of psychology in its application to mental measurements as closely as it is possible to do so within the limits of a single volume written primarily for the reader who has no special scientific training along psychological lines.