The second method we have described, viz., that of using as the test a real sample of the work done, has certain very obvious advantages. On the other hand, for the vocational test of this type to be at all significant, either the sort of work involved in the occupation must be fairly uniform and homogeneous in all its different circumstances (as in the case of typewriting at dictation, or in the work of filing clerks, accountants, etc.), or else there must be included a large number of samples representing all the various unrelated sorts of work. Moreover, in neither case is the test in any peculiar sense psychological. Such tests could perhaps be best conducted by the employer himself. In fact, employment on trial, which is a common method of selecting operatives and assistants, is a time-honored form of this test, which is not necessarily improved either by calling it psychological or by putting it in charge of a general expert or by removing it to the laboratory.
The third form of procedure is full of all sorts of difficulties and sources of error, many of which are, at the present stage of our knowledge, irremediable. In selecting a new test which will involve the same mental attitude and call for the exercise of the same psychological functions as are needed in the work itself, we are handicapped by the unreliability of the introspection of the examinee and also by our inadequate ability to recognize, identify and classify psychological functions even when we are confident that these are present. The statement of motormen that the manipulation of a crank in connection with a strip of checkered paper makes them feel quite as they do when guiding their cars through a crowded thoroughfare is far from a guaranty "that the mental function which they were going through had the greatest possible similarity with their experience on the front platform of the electric car." It is much more conceivable that the "mental attitude" referred to was merely the vague feeling that "Something is happening now," "This keeps me busy," or "What a nuisance this thing is." And even if we knew the mental functions involved, as would be demanded by the vocational psychograph method, we are still a long way from the time when we can exhibit even a single psychological test and state just what function or functions its performance does or does not, may or may not, involve. Indeed we do not even know what the various distinct mental functions are, or whether, as a matter of fact, there are such distinct functions.