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 exceptional
 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 language
 roughly equivalent to the completion of five years in
the public schools. Where many of the workers are foreigners
 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 directions
 which call for reading or for verbal written response.
The latter procedure is necessary in many American factories,
 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 correlation
 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 combination
 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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