Literature review to assemble the evidence for response scales used in patient-reported outcome measures

BACKGROUND: In the development of patient-reported outcome (PRO) instruments, little documentation is provided on the justification of response scale selection. The selection of response scales is often based on the developers’ preferences or therapeutic area conventions. The purpose of this literature review was to assemble evidence on the selection of response scale types, in PRO instruments. The literature search was conducted in EMBASE, MEDLINE, and PsycINFO databases. Secondary search was conducted on supplementary sources including reference lists of key articles, websites for major PRO-related working groups and consortia, and conference abstracts. Evidence on the selection of verbal rating scale (VRS), numeric rating scale (NRS), and visual analogue scale (VAS) was collated based on pre-determined categories pertinent to the development of PRO instruments: reliability, validity, and responsiveness of PRO instruments, select therapeutic areas, and optimal number of response scale options. RESULTS: A total of 6713 abstracts were reviewed; 186 full-text references included. There was a lack of consensus in the literature on the justification for response scale type based on the reliability, validity, and responsiveness of a PRO instrument. The type of response scale varied within the following therapeutic areas: asthma, cognition, depression, fatigue in rheumatoid arthritis, and oncology. The optimal number of response options depends on the construct, but quantitative evidence suggests that a 5-point or 6-point VRS was more informative and discriminative than fewer response options. CONCLUSIONS: The VRS, NRS, and VAS are acceptable response scale types in the development of PRO instruments. The empirical evidence on selection of response scales was inconsistent and, therefore, more empirical evidence needs to be generated. In the development of PRO instruments, it is important to consider the measurement properties and therapeutic area and provide justification for the selection of response scale type.