XIII 
VALIDATION OF THE MEASURING IN STRUMENTS 
Tabulation. Meaning of reliability. Measures of reliability. Correction for 
attenuation. Meaning of validity. Measures of validity. Group compari- 
sons. Differentiation of groups by test scores. Differentiation of groups by 
questionnaire items. 
THE psychological measurements and records of the sub- 
jects which the investigator has gathered by means of his 
tests, questionnaires, and rating scales mean nothing in 
themselves. The investigator must determine their validity, 
that is, the relationship between these measurements and 
the criterion of vocational accomplishment. Once he has 
found the extent of this relationship, he can show how far 
such measurements can be of service in selecting men for 
the vocation. 
This and the following chapter deal with the statistical 
methods of comparing measurements of abilities with the 
criterion of success. They treat of the ways of determining 
quantitatively how much relationship there is between these 
variables. 
Discovery and measurement of relationship between dif- 
ferent sets of facts is basic in all science. In the biological 
sciences it is commonly called the establishment of correla- 
tion. Ordinary experience and common sense reveal many 
relationships of dependence between one set of facts and 
another; but it is only when these facts have been reliably 
measured and a statistical index of relationship between the 
two series of measures computed, that knowledge of these 
relationships can be called scientific. Without such scien- 
tific knowledge of the interdependence of predictive mea- 
surement of ability and of actual competence on the job, any 
recommendations regarding the introduction or the abandon- 
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