decidedly the most essential factor and the one from which most can be
hoped for the safety of the public. But this does not exclude the
possibility that some other mental traits may become causes of
accidents. The man may be too daring and may like to run risks, or he
may still need discipline, or he may not be sufficiently acquainted
with the local conditions. Any such secondary factors may cause some
slight accidents with the man who shows rather fair results in the
experimental test of his foresight. Finally, we must not forget that
some men enter into such tests under a certain nervous tension and
therefore may not show so well at the very first test as their mental
equipment should allow. Hence it is decidedly desirable not to rely on
the first test, but to repeat it. If those various interferences are
taken into account, the correspondence between efficiency and the
results of the tests is fairly satisfactory. It justified me in
proposing that the experiments be continued and in regarding it as
quite possible that later tests on the basis of this principle may be
introduced at the employment of motormen.

A difficulty is presented by the valuation of the numerical results.
The mere number of omissions alone cannot be decisive, as it is clear
that no intelligent man would make any omissions if he should give an
unlimited amount of time to it; for instance, if he were to spend
fifteen minutes on those 12 cards. But this is the same thing as to
say that a motorman would not run over any one if he were to drive his
car one mile in an hour. The practical problem is to combine the
greatest possible speed with the smallest number of oversights and
both factors must therefore be considered. The subject who makes
relatively many mistakes but uses a very short time must be
acknowledged to be as good as the man who makes fewer mistakes but
takes a longer time. In the results which I have gathered in