Scale Type Revisited: Some Misconceptions, Misinterpretations, and Recommendations

Stevens’s classification of scales into nominal, ordinal, interval, and ratio types is among the most controversial yet resilient ideas in psychological and educational measurement. In this essay, I challenge the notion that scale type is essential for the development of measures in these fields. I highlight how the concept of scale type, and of interval-level measurement in particular, is variously interpreted by many researchers. These (often unstated) differences in perspectives lead to confusion about what evidence is appropriate to demonstrate interval-level measurement, as well as the implications of scale type for research in practice. I then borrow from contemporary ideas in the philosophy of measurement to demonstrate that scale type can only be established in the context of well-developed theory and through experimentation. I conclude that current notions of scale type are of limited use, and that scale type ought to occupy a lesser role in psychometric discourse and pedagogy.