EMPLOYMENT PSYCHOLOGY
If the new methods prove to be more efficient and are
recommended for adoption, he installs them as a part of the
employment procedure. He also assures himself from time
to time that they are being used properly, and checks their
predictive accuracy frequently in order to adjust them if
necessary to changes in industrial demands or type of
applicant.
Not every research in vocational selection will require that
all of these 11 steps be included in the program. The
investigator will, however, be more apt to avoid pitfalls if
he examines his outline of proposed procedure critically with
each of these steps in mind, and assures himself that provision
 has been made for every stage of investigation essential
 to the sound progress of his particular study.
We are primarily concerned with the scientific measurement
 of aptitudes for an occupation. The method outlined
may also be used -in developing differentiating tests for
placement. It can be used in distinguishing between several
occupations which differ in the degree of difficulty of their
work. The method may be employed, for example, in
validating tests for placing office help at either typewriting,
stenography, or secretarial work; or at filing or comptometer
work.
The examinations of which this manual treats in greatest
detail are psychological tests. The accuracy with which a
test measures an ability to succeed in a job varies with the
extent to which it calls out the same or similar types of
adjustments as the job. If there is a close relationship
between test scores and success at the job, disturbing variables
 being held constant, the assumption may be made that
the relationship is caused by the fact that the two call out
the same ability or abilities.
A causal connection between two variables is merely the
observed concomitant variation of the two. A causal relationship
 between test performance and success at the job
may be inferred if the correlation between them is high.
Even this modicum of theory may be omitted. If the corre-Q