Self-Evaluation, Differential Item Functioning, and Longitudinal Anchoring Vignettes

Anchoring vignettes are a powerful instrument to detect systematic differences in the use of self-reported ordinal survey responses. Not taking into account the (non-random) heterogeneity in reporting styles across different respondents may systematically bias the measurement of the variables of interest. The presence of such individual heterogeneity leads respondents to interpret, understand, or use the response categories for the same question differently. This phenomenon is defined as differential item functioning (DIF) in the psychometric literature. A growing amount of cross-sectional studies apply the anchoring vignette approach to tackle this issue but its use is still limited in the longitudinal context. This chapter introduces longitudinal anchoring vignettes for DIF correction, as well as the statistical approaches available when working with such data and how to investigate stability over time of individual response scales.