EMPLOYMENT PSYCHOLOGY
ment of specific employment tests or other indexes of probable
 success cannot command the respect of the thoughtful
business executive who has learned to appreciate science not
only in engineering and finance but also in the realm of
human adjustment. The procedure of determining the relationship
 between measurements of ability and the criterion
of vocational success requires an understanding of what is
meant by coefficients of correlation between two variables,
by reliability of the measures on which these correlations are
computed, and by validity of the test measurement as judged
by its correlation with the criterion. The reader must turn
to treatises on statistical method mentioned below for a comprehensive
 discussion of theory. Here he will find conveniently
 summarized the successive statistical steps to be
taken in ascertaining the significant relationships between
test score and criterion. The methods of determining the
predictive value of definite scores will be reserved for later
chapters.
There are conditions, to be determined for each investigation,
 under which the methods and devices for test validation
 may be omitted entirely and the investigator may
proceed at once to the practical methods described in Chapters
 XV and XVI for determining the predictive value of
each score. If, for example, distribution curves of test
scores have been drawn for vocational successes and for
vocational failures, critical scores or critical sections (preferred
 ranges) may be set off even though the significance of
the difference between the two groups in test performance
has not been determined.
It will be assumed in this discussion of statistical devices
that the reader is familiar with one or more of the standard
text-books on statistics, such as Rugg (157), Thurstone
(196), Otis (125), Yule (233), West (219), Brown and
Thomson (21), Jones (83), Thorndike (191), Kelley (86),
and Davenport (42); and that he knows the meaning and
significance of the most common statistical concepts. The
main purpose of these chapters is to outline all the statistical

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