CHOICE OF WORKERS TO BE STUDIED 51 
criterion and tests and on the conditions of the investiga- 
tion. 
The investigator must make certain that all of his sub- 
jects have achieved their standing in the criterion by the 
exercise of those abilities considered necessary for success 
at the job, and not through chance circumstances or special 
advantages, 
DISTURBING VARIABLES 
In selecting the workers to be measured, extreme varia- 
tion in age or any other gross disturbing variables which 
are apt to have an effect on test performance should not be 
allowed to complicate the problem. These variables should 
be controlled by spreading them evenly throughout the dis- 
tribution of classes of vocational accomplishment, or by 
eliminating as subjects those who represent extremes in the 
disturbing variables. 
Examples of a few of these variables may be cited. 
Sex. If men and women are both employed in the occu- 
pation, it will be necessary to examine the results for sex 
differences, and if need be, to evaluate the tests separately 
for the two sexes. The importance of this point will be made 
obvious if the reader will look into the literature on sex 
differences in test performance (see 73, p. 112). In such a 
test as card sorting only 24% of the men reach the median 
score of the women, whereas in speed of movement (finger 
and arm) 71% of the men reach the median score of the 
women, 
Age. Differences in the ages of the subjects should not 
be so great as to have an influence on their comparative 
test performances. If proper sampling does not permit a 
limited range of ages, then the data should be examined to 
see if some correction should be made for great differences 
in age. 
Sensory defects. If sensory acuity is a factor in success 
in the vocation, it should be determined in a special exam-