However, the laboratory psychologists nowadays have gone far beyond such superficial claims for correlations of symptoms.  With experimental and statistical methods they have gathered ample material which demonstrates the exact degree of probability with which we have a right to expect that certain qualities will occur together.  Theoretically we may take it for granted that those traits which are always present together or absent together ultimately have a common mental root.  Yet practically they appear as two independent traits, and therefore it remains important to know that, if we can find one of them, we may be sure that the other will exist there too.  Inasmuch as the one of the two traits may be easily detected, while the other may be hidden and can be found out only by long careful tests, it would be valuable, indeed, for the employment manager to become acquainted with such correlations as the psychologist may discover:  as soon as he becomes aware of the superficially noticeable symptom, he can foresee that the other disposition is most probably present.  To give an illustration:  in the interest of such measurements of correlations we have studied in the Harvard laboratory the various characteristics of attention and their mutual dependence.[18] We found that typical connections exist between apparently independent features of attention.  Persons who have a rather expansive span of attention for acoustical impressions have also a wide span for the visual objects.  Persons whose attention is vivid and quick have on the whole the expansive type of attention, while those who attend slowly have a narrow field of attention, and so on.  Hence the manifestation of one feature of attention allows us to presuppose without further tests that certain other features may be expected in the particular individual.

The problem of attention, indeed, seems to stand quite in the centre of the field of industrial efficiency.  This conviction has grown upon me in my observation of industrial life.  The peculiar kind of attention decides more than any mental trait for which economic activity the individual is adapted.  The essential point is that such differences of attention cannot be characterized as good or bad; it is not a question of the attentive and of the inattentive mind.  One type is not better than another, but is simply different.  Two workingmen, not only equally industrious and capable, but also equally attentive, may yet occupy two positions in which they are both complete failures because their attention does not fit the places, and both may become highly efficient as soon as they exchange positions.  Their particular types of attention have now found the right places.  The one may be disposed to a strong concentration by which everything is inhibited which lies on the mental periphery, the other may have the talent for distributing his attention over a large field, while he is unable to hold it for a long while at one point.  If the one industrial