The visual analog rating scale of health-related quality of life: an examination of end-digit preferences

BACKGROUND: The Visual Analog Scale (VAS) has been extensively used in the valuation of health-related quality of life (HRQL). The objective of this paper is to examine the measurement error (rounding) explanation for the higher prevalence of VAS scores ending with a zero, and to provide an alternative interpretation. METHODS: The analysis is based on more than 4,500 reported VAS valuations of own HRQL, included in two Israeli health surveys (1993 and 2000). Bivariate and logistic regression analyses are used. RESULTS: The results show that reporting VAS scores ending with a 0 (...-20, ..0,10,20.....) decreases and scores ending with a 5 (...-15,-5,5,15,25,...) and with any other integer (...-12, -11,...1,2,...,92,..99) increases as VAS scores depart from 50, particularly when increasing up to 100. This pattern remains after controlling for personal characteristics determining the level of VAS. DISCUSSION: Rounding true HRQL to the nearest 10's or 5's cannot explain the specific pattern found. It is suggested that this pattern corresponds to a S-shaped value function, where individuals tend to evaluate their HRQL as "gains" or "losses" relative to a reference point evaluated at 50. This particular reference score originates from being a traditional "passing threshold" and the scale's midpoint. Several implications of this interpretation to the measurement of HRQL are discussed.