Measurement: a beginner's guide

This paper provides an introduction to measurement theory for psychometricians. The central concept in measurement theory is that of a continuous quantitative attribute and explaining what measurement is requires showing how this central concept leads on to those of ratio and real number and distinguishing measurements from measures. These distinctions made, the logic of quantification is described with particular emphasis upon the scientific task of quantification, as opposed to the instrumental task. The position presented is that measurement is the estimation of the magnitude of a quantitative attribute relative to a unit and that quantification is always contingent upon first attempting the scientific task of acquiring evidence that the relevant attribute is quantitative in structure. This position means that the definition of measurement usually given in psychology is incorrect and that psychologists' claims about being able to already measure psychological attributes must be seriously questioned. Just how the scientific task of investigating whether psychological attributes are quantitative may be undertaken in psychology is then considered and the corollary that psychological attributes may not actually be quantitative is raised.