Introduction

That the conceptual problems inherent in measurement in general, and psychological measurement in particular, are poorly understood is obvious from the lack of agreement on the meaning of the term ‘measurement’, the multitude of conceptually different models for implementing it, and the fact that no psychologist can point to a field where psychological measurement has succeeded without eliciting an immediate claim to the contrary from another psychologist. Given that virtually all aspects of the measurement problem are the subject of ongoing debates, many of which have been discussed in previous chapters, one would expect these debates to culminate in fierce discussions on the most central question one can ask about psychological measurement, which is the question of validity. It is therefore an extraordinary experience to find that, after proceeding up through the turmoil at every fundamental level of the measurement problem, one reaches this conceptually highest and presumably most difficult level only to find a tranquil surface of relatively widespread consensus (Kane, 2001; Shepard, 1993).

A remarkable aspect of this consensus in validity theory is that the concept with which validity theorists are concerned seems strangely divorced from the concept that working researchers have in mind when posing the question of validity.