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
experiments with motormen, no one has gone through those 12 cards in a
shorter time than 140 seconds, while the longest time was 427 seconds.
On the other hand, no one of the motormen made less than 4 omissions,
while the worst ones made 28 omissions. I abstract from one extreme
case with 36 omissions. On the whole, we may say that the time
fluctuates between 180 and 420, the mistakes between 4 and 28. The aim
is to find a formula which gives full value to both factors and makes
the material directly comparable in the form of one numerical value
instead of the two. If we were simply to add the number of seconds and
the number of omissions, the omissions would count far too little,
inasmuch as 10 additional omissions would then mean no more than 10
additional seconds. On the other hand, if we were to multiply the two
figures the omissions would mean by far too much, as the transition
from 4 mistakes to 8 mistakes would then be as great a change as the
transition from 200 to 400 seconds, that is, from the one extreme of
time to the other.
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