2 EMPLOYMENT PSYCHOLOGY 
rainstorm knocks the rear wheel off another car is obviously not 
so much the cause of the accident as one who has the same sort 
of accident on a bright, clear day. Likewise, icy pavements are 
often the cause of accidents which are inexcusable in better 
weather. What is careful and slow driving in one case will not 
be so in the other. That, too, must be considered. 
There are even other complicating factors, such as a careless 
pedestrian stepping in front of the cab. Here the driver is not 
the cause, yet technically he has had an accident. There may be 
some unknown defect in the car mechanism which may cause an 
accident. Again the driver is not the cause, yet technically he 
has had an accident. 
These, then, are the variables: To what degree is the driver 
the cause of the accident? Should this be modified by the time 
factor, or by the weather conditions? These are very pertinent 
questions which must be dealt with in establishing criteria with 
which to standardize the tests, to test them (correlate them with 
actual fact), and to prorate them (determine their weight in 
establishing a final score). 
Even if we determine whether or not the driver is the cause, 
it is impossible to discriminate finely the extent to which he is 
the cause. This cannot be expressed numerically, nor can it even 
be accurately expressed with a rating scale. 
The legal department of the company, since it gets all the 
original information—reports from the passengers and witnesses, 
as well as from the drivers—attaches one of the three following 
opinions to each accident: gross negligence, unavoidable, question- 
able. Tt is this judgment we use in determining to what extent 
the driver is the cause of the accident. 
Snow’s success in applying the psychology of vocational 
selection to the problem of choosing cab operators rests not 
wholly on his ingenuity in devising and adapting tests. His 
critical scrutiny of his criteria and his strict adherence to 
them in planning his research and in evaluating its results 
have likewise been important. 
A criterion may be expressed in terms of a rough twofold 
or threefold classification of the workers, or in terms of 
such fine units of measurement that each man is distin- 
guished from the others in accomplishment. If the criterion, 
for example, is the amount sold in the course of a year, 
the men may be separated into two groups: those who sold 
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