Reliability is not validity and validity is not reliability

Interrater correlations do provide an index of reliability of job performance ratings. We show that the arguments presented by Murphy and DeShon (2000) lead to the radical conclusion that traditional measurement models-both classical theory and generalizability theory models-can be used neither with job performance ratings nor with other measures used in I-O and other areas of psychology and the social sciences. We show that this untenable conclusion is based on confusion of validity issues and questions with reliability issues and questions. It is also based on the incorrect belief that classical measurement models are capable of addressing only random response measurement error and cannot address other forms of measurement error. We also show that the solution Murphy and DeShon offer to the problem of measurement error in ratings, as they define this problem, cannot work. Properly understood, the position taken by Murphy and DeShon leaves us with the nihilistic conclusion that no appropriate measurement models are possible in psychological research, thus making meaningful research impossible.