How Should Classical Test Theory Have Defined Validity?

Classical test theory defined the predictive validity of a test as the ordinary Pearson correlation between scores on the test and scores on a validation criterion. For some purposes this definition is satisfactory, but for others it leads to complications, because derivation of familiar equations relating validity and reliability requires an independent assumption of uncorrelated errors of measurement. The present paper proposes an alternate definition of validity that avoids difficulties arising from correlated error scores and is more consistent with standard definitions of true score, error score, and reliability in the classical theory.