Introduction

Psychological assessment is subject to various errors of measurement. While some are random, as assumed in classical test theory, others are systematic and lead to consistent distortions of the true value of a characteristic. These latter errors may be partially due to assessor's biases. This term does not refer to elementary professional mistakes such as implementing test instructions incorrectly, but to systematic tendencies in case-related information processing that reduce the validity of data. Although these biases normally impair objectivity and reliability, they remain undetected when they are consistent across individuals and time. In addition, a low interrater agreement is not necessarily a sign of assessor's bias but may be due to valid differences between settings and informants (Lösel, 2002).

Not all types of assessment information are ...