Why Psychometrics is Not Pathological

This paper comments on an article by Michell (2000), who argues that psychometrics should be qualified as pathological science for two reasons: (a) psychometrics assumes psychological attributes to be quantitative without testing this hypothesis; and (b) the fact that this hypothesis is not tested is disguised. Michell further argues that the hypothesis should be tested using additive conjoint measurement theory. Although relevant to classical test theory, Michell’s arguments do not apply to psychometrics in general. In particular, they are largely irrelevant to item response theory models. We show that these models result from introducing probabilistic relations, which are needed to deal with measurement error, and not from a breakdown in critical inquiry, as Michell suggests. Moreover, at least one class of these models can be formulated in terms of additive conjoint measurement theory, which renders Michell’s call for the additive conjoint model in need of qualification. Finally, item response theory models are routinely tested against empirical data, and although the assumption that an attribute is quantitative cannot be tested directly, such tests do address the conjunction of this assumption and other model assumptions. We conclude that, although Michell’s arguments are important to psychological measurement, they are largely irrelevant to item response theory. In fact, we argue that they can be phrased in terms of this theory in a natural way.