The principal distinction, scientifically, between a trade test and an intelligence test, is in the purpose to be served by the test. In the intelligence test the aim is to ascertain the subject’s general _capacity_; in the trade test, to discover his present _ability_ or degree of skill in some special direction. Capacity, as has been previously pointed out, is only to be measured in terms of demonstrable ability, so that in the application of trade tests, although limited in their scope to a single class or kind of ability, there is also obtainable as a by-product a partial measure of the subject’s mental capacity. While trade tests devised by psychologists had been demonstrated, in a number of industries, to be superior to any other method, both in picking the most skilful workers from among all applicants for positions, and in transferring workers from one department to another in large industries, it was in the classification and placement of the personnel of the Army during the war that the first really large-scale demonstration of the precision and effectiveness of scientifically devised trade tests was made. While one group of psychologists, working under the direction of the Surgeon-General’s Office, was engaged in classifying the Army personnel by means of intelligence tests the Personnel Branch of the Operations Division of the General Staff, organized and officered by trained psychologists, was undertaking the task of determining the special technical and vocational ability of the millions of men drawn into the Army through the medium of the selective draft. This personnel organization had a multiplex duty to perform. First, it had to ascertain with precision what particular kinds of work had to be done in the preparation of an army for battle and in its transport and maintenance. This involved not only finding out just what needed to be done but translating this need into terms of trades and occupations. For example, the Army might report that it needed a number of men capable of making all sorts of repairs to electric generators and motors. The Personnel Division proceeded to analyze the special qualifications required of electricians to enable them to meet this demand. These were listed, along with the qualifications required for every other army occupation, in a thick book entitled Trade Specifications Index. There were 239 pages in this book and in it were set forth in specific detail the exact qualifications needed by 565 different kinds of trade and technical experts. Chauffeurs, for instance, were classed as auto drivers, auto drivers with pigeon experience, motorcycle drivers with pigeon experience, plain motorcyclists, heavy auto-truck drivers, motor truck drivers, and plain chauffeurs. There were sixteen different classes of electricians, each of which required a man with special experience and knowledge. Nine different kinds of chemists were used in the Army.