An introduction to the Rasch measurement approach for metrologists

In the interests of fostering an inter-disciplinary dialogue, increasing collaboration between "hard" and "soft" measurement scientists, and learning from one another, the paper develops an analytical discussion of common elements between metrology and psychometrics. A simple example of physical measurement is introduced according to the conceptualization and terminology of the International Vocabulary of Metrology (VIM), and then its structural analogy to a test using Guttman items is shown. On this ground the example is generalized so to include a probabilistic component, and this leads to the basic Rasch model. Some notes on the delicate issue of measurement validity conclude the paper, whose aim, in the long run, is a measurement-related shared concept system, and a terminology understandable in both physical and social sciences