High-quality, informative research in clinical psychology depends on the use of measures that have sound psychometric properties. Reliance upon psychometrically poor measures can produce results that are misleading both quantitatively and conceptually. This chapter articulates the implications that psychometric quality has for clinical research, outlines fundamental psychometric principles, and presents recent trends in psychometric theory and practice. Specifically, this chapter discusses the meaning and importance of measures’ dimensionality, reliability, and validity, and outlines the diverse methods for evaluating those important psychometric properties. In doing so, it highlights the utility of procedures and perspectives such as confirmatory factor analysis, exploratory structural equation modeling, classical test theory, item response theory, and contemporary views on validity. It concludes with a brief comment about the process of creating and refining clinical measures. The chapter’s ultimate goal is to enhance researchers’ ability to produce high-quality and informative clinical research.

In this chapter, we address the key psychometric concepts of standardization, reliability, validity, norms, and utility. In doing so, we focus primarily on classical test theory (CTT) – the psychometric framework most commonly used in the clinical assessment literature – which disaggregates a person’s observed score into true score and error components. Given its growing use with psychological instruments, we also present basic information on aspects of item response theory (IRT). In contrast to CTT, IRT assumes that some test items are more relevant than other items for evaluating a person’s true score and that the extent to which an item accurately measures a person’s ability can differ across ability levels. After presenting the central aspects of these two frameworks, we conclude the chapter with a discussion of the need to consider cultural/diversity issues in the development, validation, and use of psychological instruments.