Abstract
Item response theory (IRT) and classical test theory (CTT) are invaluable tools for the construction of assessment instruments and the measurement of student proficiencies in educational settings. However, the advantages of IRT over CTT are not always clear. This chapter uses an example item analysis to contrast IRT and CTT. It is hoped that the readers can gain a deeper understanding of IRT through comparisons of similarities and differences between IRT and CTT statistics. In particular, this chapter discusses item properties such as the difficulty and discrimination power of items, as well as person ability measures contrasting the weighted likelihood estimates and plausible values in non-technical ways. The main advantage of IRT over CTT is outlined through a discussion on the construction of a developmental scale on which individual students are located. Further, some limitations of both IRT and CTT are brought to light to guide the valid use of IRT and CTT results. Lastly, the IRT software program, ConQuest (Wu et al. ACERConQuest version 2: Generalised item response modelling software. Australian Council for Educational Research, Camberwell, 2007), is used to run the item analysis to illustrate some of the program’s functionalities.
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