The Janus Face of Psychometrics

Most psychometric data are behavioral data: responses to cognitive problems and to questionnaire items referring to behavior in a direct or indirect way. Therefore, measurement models are at the same time psychological models. The Janus face metaphor refers to these two sides of psychometrics. Measurement models can fail as psychological models. We discuss three examples, called vignettes in this chapter. The first refers to reflective measurement models not being in line with the psychology of what is measured. The second example concerns measurement invariance and the psychological meaningfulness of measurement invariance violations. The third example refers to the error variance (unexplained variance) in measurement models and models in general and how the error may be explained by individual-specific psychological phenomena.