Classical test theory (CTT) is widely used in psychological assessment and has a long history. Its origins as a scientific method of inquiry can be traced back to the nineteenth century. The assessment of mindfulness and related psychological constructs, such as compassion, is heavily reliant upon self-report, reflecting the highly subjective or existential nature of such phenomena. To enable quantitative examination of such intangible subject matter, psychologists have traditionally used self-report questionnaires to operationalize latent variables. The research methods and statistical techniques developed to ensure reliable and valid measurement of educational (e.g., reading skills) and psychological constructs (e.g., extraversion) by questionnaire have become known collectively as psychometrics. The modern psychometrician has a rich array of statistical methods and software tools to choose from, and CTT remains prominent among them. This includes the widely used methods of exploratory and confirmatory factor analysis.