Thus, opposite conditions lead to maximization of Equations 7.1 and 7.2. Equation
7.1 shows that internal consistency is maximized when items are highly correlated, and Equation 7.2 shows that criterion-related validity is maximized when items are uncorrelated. In practical terms, this means it is mathematically impossible to simultaneously maximize reliability and validity of a scale.
Therefore, test developers must choose between making very homogeneous scales
that reliably predict only a narrow set of behaviors versus creating more heterogeneous scales that measure more comprehensive scale content. Because the predictive validity of a scale is the ultimate measure of its worth, internal consistency reliability should not be the main criterion used in scale development.