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.
Two independent developments in test theory appeared in the 1960s, one articulated by Rasch, 1 the other by Lord and Novick 2 and Birnbaum. 3 This theory is now generally referred to as item response theory (IRT). Applications of these models have more recently been extended into medical and health care assessment. 4–7
In education, as now in other fields, these models were seen as especially useful in assessing performance and achievement across groups using conformable items in which not all persons needed to respond to all items. However, 2 perspectives emerged. In one, Rasch models are only special cases of IRT models; in the other, they are essentially different. These perspectives, still evident in the literature, 7 originally generated controversy. Proponents of the 2 perspectives are now more likely to agree to disagree, but the perspectives continue to manifest themselves in reviews of articles, which in turn reflect what, with what emphasis, and where various studies are published, and in presentations of articles from one perspective without presentations from the other. New researchers reading the literature and applying IRT to new fields can find these differences puzzling. It seems relevant, therefore, to understand the 2 perspectives. This paper attempts to do so by considering the original controversy. The approach taken is to compare the 2 perspectives using the framework of the physicist and historian of science, Thomas Kuhn 8 at 3 levels: first, his concept of research paradigm within scientific revolutions; second, his recognition of roles of individual histories of scientists; and third, his explanation of the role of measurement in physical science.