h the 12 cards.
The experimenter notes down the numbers of the cards and the letters
which the subject calls. Besides this, the number of seconds required
for the whole experiment, from the beginning of the first card to the
end of the twelfth, is measured with a stopwatch. This time is, of
course, dependent upon the rapidity with which the crank is turned.
The result of the experiment is accordingly expressed by three
figures, the number of seconds, the number of omissions, that is, of
places at which red figures would land on the track which were not
noticed by the subject; and, thirdly, the number of incorrect places
where letters were called in spite of the fact, that no danger
existed. In using the results, we may disregard this third figure and
give our attention to the speed and the number of omissions.

The necessary condition for carrying out the experiments with this
apparatus is a careful, quiet, practical explanation of the device.
The experiment must not under any circumstances be started until the
subject completely understands what he has to do and for what he has
to look out. For this purpose I at first always show the man one card
outside of the apparatus and explain to him the differences between
the black and the red figures, and the counting of the steps, and show
to him in a number of cases how some red figures do not reach the
track, how others go beyond the track, and how some just land in
danger on the track. As soon as he has completely understood the
principle, we turn to the apparatus and he moves the window slowly
over a test card, and tries to find the dangerous spots, and I turn
his attention to every case in which he has omitted one or has given
an incorrect letter. We repeat this slowly until he completely masters
the rules of the game. Only then is he allowed to start the
experiment. I have never found a man with whom this preparation takes
more than a few minutes.

After developing this method in the psychological laboratory, I
turned to the study of men actually in the service of a great electric
railway company which supported my endeavors in the most cordial
spirit. In accordance with my request, the company furnished me with a
number of the best motormen in its service, men who for twenty years
and more had performed their duties practically without accidents,
and, on the other hand, with a large number of motormen who had only
just escaped dismissal and whose record was character