LAUSR

BACKGROUND The Faces Pain Scale-revised (FPS-r) has been developed as an interval scale. For other pain measurement instruments, several studies found evidence for and against an interval level of measurement. OBJECTIVES The primary aim of the current study was to evaluate the scale properties of the FPS-r using an item response theory approach. DESIGN Secondary analysis of published data. SETTING Three studies; Study 1 and study 2: One university hospital; Study 3: international pain registry. PARTICIPANTS Study 1: n = 246, female: 41%, age: 11-18 years, 3 pain items; Study 2: n = 240, female: 43%, age: 11-18 years, 9 pain items; Study 3: n = 2266, female: 41%, age: 4-18 years, 3 pain items. METHODS The rating scale model (interval scale), the graded response model (no interval scale, ordered response categories) and the partial credit model (no interval scale) were used to scale the data. RESULTS In all three studies, the rating scale model was outperformed by the graded response model or the partial credit model in terms of model fit. Overlapping response categories were found in items associated with less pain. Response category widths were wider for categories associated with low pain intensity and smaller for categories associated with high pain intensities. Smallest response categories were 1%-67% smaller compared to the widest response category of the same item. CONCLUSION According to these findings, the interval scale properties of the FPS-r may be questioned. Item response theory methods may help to solve the problem of missing linearity in pain intensity ratings using FPS-r.