A fundamental principle of personality measurement

" . . . an individual reveals his own personality through any change he makes upon any type of material . . . . Any material will itself limit the range of possible responses. It is therefore likely that it will be of relatively little value for measuring some aspects of personality, even though it is of great value in measuring others. If personality is to be measured, the changes or the behavior which the material evokes must satisfy the usual test criteria of reliability and validity . . . . The principle . . . forces us to think more clearly concerning two aspects of personality measurement . . . 1. The range of possible test materials is almost infinitely large . . . . 2. Which materials within this large range can be employed depends upon how satisfactorily they satisfy the well-established statistical criteria of test construction."