CRITERIA OF VOCATIONAL SUCCESS 23 
the driver to avoid. The car maintenance department attempts 
to minimize those due to mechanical defects. 
If the purpose of the investigator is to reduce the number 
of accidents by better selection, he must use as many cri- 
teria as there are types of accidents. He has to construct 
tests independently for measuring the ability required to 
avoid each type of accident. It is clear that one test or a 
battery of tests cannot be expected to predict accidents of 
all types. One test may predict accidents due to emotional 
instability and another test accidents due to some physio- 
logical defect. 
Snow used each of the first five types of accidents as his 
criteria for the evaluation of tests. But he was still faced 
with difficulties. To quote his own words: 
The accident data on each driver are obtained in this way: 
When a driver has an accident it is recorded against him. All 
accidents, however, are not equal in the degree to which the driver 
is the cause. There are several variables. The driver contributes 
to some accidents to a greater degree than to others, For 
instance, a driver hits a telegraph pole which is rotted; the pole 
breaks and no one is injured. Another driver has the same sort 
of accident, except that the pole he strikes is not at all rotted and 
his passenger is injured. The consequences are different, but the 
cause is the same. 
Then there is the factor of time—the length of service of the 
man, and his previous experience. One driver in his first two 
months has, say, two accidents, while another has but one acci- 
dent in three months, then has three in the next two weeks. In 
such a case, one must remember that some drivers have accidents 
relatively often at the start, but relatively seldom after a short 
period. The reason may be that these men have never driven 
before, or that it takes them longer to adjust themselves to the 
new conditions, but once they do learn they make very good 
drivers. 
On the other hand, some men have relatively no accidents at 
the start, but later develop a tendency toward them. Perhaps 
they are quick learners, or have had previous experience in driv- 
Ing, and once the newness of the position is worn off, they soon 
let up and become less careful. But how is one to classify and 
evaluate accidents under such conditions? 
There is also a weather factor. A driver who during a heavy 
ic