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Valuing health states using VAS and TTO: what lies behind the numbers?

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It is well known that different methods of eliciting the valuations attached to various health states, such as the Visual Analogue Scale (VAS) and the Time Trade Off (TTO), yield different results. This study gathers qualitative data from a group of 43 respondents who had previously taken part in a large scale national study which set out to elicit the values attached by individuals to various health states using both the VAS and the TTO techniques. The findings of this study raised three questions which are of particular interest here: (1) Why are some states that are rated better than dead on the VAS often rated as worse than dead in TTO? (2) Why are some respondents unwilling to trade off any time at all in order to avoid a health state that they place below full health on the VAS? (3) Why are TTO valuations of older respondents for the more severe health states lower than those of the younger age groups? This study has uncovered qualitative evidence on each of these three key issues. Regarding the first question, many respondents did not appear to interpret a better than dead VAS score as a strict preference for spending 10 years in a health state over immediate death. Several different factors appeared to contribute towards this, an important one being the tendency of respondents to ignore the duration of the health state during the VAS task. Regarding the second question, there is evidence of the existence of a “threshold of tolerability” below which states would have to fall before some respondents would be willing to give up any time at all on the TTO. Regarding the last question, it appears that older respondents are less likely to find the worse than dead TTO scenario plausible than those in the younger age groups. However, whilst this may explain why older respondents attach lower worse than dead valuations to health states, it does not appear to account for the entire difference in TTO valuations between the two age groups. In addition, it appears that older respondents may be less prepared to live for the next 10 years in a diminished health state.

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