Multidimensional unipolar IRT and applications to the measurement of print exposure

Item response theory (IRT) has been a prominent modelling framework in educational and psychological measurement. Traditional IRT models are bipolar, commonly assuming symmetric measurement link functions and symmetric trait distributions over a latent continuum unbounded at both ends. However, many measured constructs, such as print exposure, vocabulary knowledge, or addictive behaviours, have been argued to be unipolar. While prior unipolar applications have emphasized the potential meaningfulness of measurement metrics having fixed lower bounds, a less-explored feature relates to how the unipolar framework has the potential to represent increased diversification of the construct as the construct level increases. This paper illustrates this feature by considering a form of multidimensional unipolar IRT modelling developed using an Accelerated Failure Time (AFT) framework. Using empirical data from the Author Recognition Test-Genres (ART-G), we show how multidimensional unipolar models can help represent such construct diversification. In the context of reading assessment, we show how such a representation has the potential to explain the diversification of vocabulary mastery as the vocabulary proficiency develops.