Using Rasch measurement for instrument rating scale refinement

Assessments with ratings in ordered categories have become ubiquitous in health, biological and social sciences. Ratings are used when a measuring instrument of the kind found in the natural sciences is not available to assess some property in terms of degree - for example, greater or smaller, better or worse, or stronger or weaker. The handling of ratings has ranged from the very elementary to the highly sophisticated. In an elementary form, and assumed in classical test theory, the ratings are scored with successive integers and treated as measurements; in a sophisticated form, and used in modern test theory, the ratings are characterized by probabilistic response models with parameters for persons and the rating categories. Within modern test theory, two paradigms, similar in many details but incompatible on crucial points, have emerged. For the purposes of this article, these are termed the statistical modeling and experimental measurement paradigms. Rather than reviewing a compendium of available methods and models for analyzing ratings in detail, the article focuses on the incompatible differences between these two paradigms, with implications for choice of model and inferences. It shows that the differences have implications for different roles for substantive researchers and psychometricians in designing instruments with rating scales. To illustrate these differences, an example is provided.

The development of Rasch models in educational and psychologic measurement in the 1960s coincided with the introduction of other similar models, now described as models of item response theory (IRT). The application of IRT models has now extended to other social sciences, including health. Originally, there was substantial controversy between those who saw Rasch models as simply special cases of IRT models and those who saw them as essentially different. Because these different perspectives continue to manifest themselves in various ways, it seems relevant to understand the source of the original controversy. This paper attempts to do so by invoking Kuhn's studies in the history and philosophy of science at 3 levels. First, it suggests that the 2 perspectives reflect Kuhn's concept of legitimate, incompatible paradigms in which controversy is a typical manifestation. Second, because Kuhn recognizes individual histories in the development of paradigms, Rasch's own shift in perspective is summarized. Third, because proponents of the Rasch models emphasize the models' compatibility with fundamental measurement found in physical science, an analogy is made between how Kuhn explains the role of measurement in the physical sciences and how proponents of Rasch models explain the role of these models in the social sciences. In particular, these roles cannot be gleaned from textbooks in science and statistics, respectively.