This paper discusses the use of anchor tests (scaling tests) to bring two or more sets of scores to a common scale. Particular attention is given to the rescaling of school based assessments against an external test or examination and to potential sources of bias in this procedure. The need for routine validity checks is emphasised, and a latent trait approach to constructing a statistical framework for tests and examination score equating is described and illustrated. Bias caused by rescaling school assessments against an inappropriate anchor test is illustrated using a 1984 attempt to rescale students assessments in English against the Australian Scholastic Aptitude Test.