CHOICE OF WORKERS TO BE STUDIED 53 
Motor defects. Organic or functional motor disturbances, 
such as tics, should receive the same attention as sensory 
disturbances. If occasionally a successful employee is 
found who suffers from such disturbances, he should be 
recognized as an exception to the general rule of health 
among the employees. If the subjects of the experiment 
are to be representative of the great number, such an excep- 
tional individual may safely be ignored. 
Literacy. Needless to say, if the tests require a language 
response, no one who is illiterate should be chosen as a 
subject. Practical literacy means more than ability to 
write one’s name. It means command of the English lan- 
guage roughly equivalent to the completion of five years in 
the public schools. Where many of the workers are for- 
eigners or otherwise not sufficiently familiar with written 
English, the investigator must either exclude them from 
among his subjects by a preliminary test of reading ability, 
or else make special adaptations of any test forms or direc- 
tions which call for reading or for verbal written response. 
The latter procedure is necessary in many American fac- 
tories, where a large fraction of the competent employees 
are apt to be unfamiliar with written English. 
While differences in literacy, motor defects, age, and sex 
have been emphasized, the investigator will be alert for 
other variables, such as extremes of health or education. 
The point cannot be stressed too much that if these gross 
disturbing variables have an influence on test performance 
they should be nullified by elimination. If this step is 
impossible, then the method of partial correlation may be 
employed to make the required correction; but it must be 
borne in mind that the necessity for the use of partial cor- 
relation in making corrections is often an indication of a 
poorly planned experiment. Use is sometimes made of 
multiple and partial correlation to obtain the best combina- 
tion of scores; but this, however, is a different matter from 
the elimination of gross variables. Multiple and partial 
correlation will be explained in Chapter XVII. 
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