Reliability is not validity and validity is not reliability

Interrater correlations do provide an index of reliability of job performan ce ratings. We show that the arguments presented by Murphy and DeShon (2000 ) lead to the radical conclusion that traditional measurement models-both c lassical theory and generalizability theory models-can be used neither with job performance ratings nor with other measures used in I-O and other area s of psychology and the social sciences. We show that this untenable conclu sion is based on confusion of validity issues and questions with reliabilit y issues and questions. It is also based on the incorrect belief that class ical measurement models are capable of addressing only random response meas urement error and cannot address other forms of measurement error. We also show that the solution Murphy and DeShon offer to the problem of measuremen t error in ratings, as they define this problem, cannot work. Properly unde rstood, the position taken by Murphy and DeShon leaves us with the nihilist ic conclusion that no appropriate measurement models are possible in psycho logical research, thus making meaningful research impossible.