Interpretation of many-valuedness in quality-of-life instruments

Assessment instruments for functioning in general and quality of life in particular often involve discrete scales with three, four, or five values, or Visual Analogue Scales (VAS) with a range 0–100. VAS scales often need to be downsized and discretized using intervals or clusters. Cutoff points for these intervals/clusters need to be carefully selected and justified. Our objective was to underline the importance of providing clear interpretations of many-valuedness appearing in quality-of-life (QoL) instruments, and to present a methodology for the provision of such clearness. Doing so, we view QoL scales as originating within the World Health Organization (WHO).