In this ITEMS module the authors provide a two-part introduction to the topic of reliability from the perspective of classical test theory (CTT). In the first part, which is directed primarily at technical beginners, they review and build on the content presented in the original didactic ITEMS article by Traub & Rowley (1991). Specifically, they discuss the notion of reliability as an intuitive everyday concept to lay the foundation for its formalization as a reliability coefficient via the basic CTT model. They then walk through the step-by-step computation of key reliability indices and discuss the data-collection conditions under which each is most suitable. In the second part, which is directed primarily at intermediary learners, the authors present a distribution-centered perspective on the same content. They discuss the associated assumptions of various CTT models ranging from parallel to congeneric, and review how these affect the choice of reliability statistics. Throughout the module, they use a customized Excel workbook with sample data and basic data manipulation functionalities to illustrate the computation of individual statistics and to allow for structured independent exploration. In addition, they provide quiz questions with diagnostic feedback as well as short videos that walk through sample solutions. 

Key words:  reliability; classical test theory; KR-20; KR-21; Cronbach’s; Pearson correlation; Spearman-Brown formula; parallel model; tauequivalent model; congeneric model