What is the difference between the VAS, TTO and DCE techniques? Can they all be used to generate a quality adjusted life year (QALY) score?
The time trade-off (TTO), visual analogue scale (VAS) and discrete choice experiment (DCE) value sets differ in the technique used to elicit the values for the models.
In the TTO task, respondents are asked to imagine they live in a certain suboptimal health state for 10 years and then to specify the amount of time they would be willing to give up to live in full health instead. For example, someone might consider that 8 years in full health is equivalent to 10 years in the suboptimal health state.
The VAS technique asks people to indicate where they think a health state should be positioned on a vertical thermometer-like scale ranging from best imaginable health to worst imaginable health.
The DCE asks people to choose between two (or more) health states of varying severity across the five EQ-5D health domains (e.g. which is better, state A  or state B ?).
There has been much discussion about the theoretical and empirical properties of these three methods, and whether it is justified to calculate QALYs using values based on these methods, although all three techniques have been used to calculate QALYs. Currently, EQ-5D-5L value sets which are developed using the EQ-VT platform are based either on a combination of TTO and DCE values, or on TTO values only.
Please note that this is a large subject area and the brief answer provided above includes minimal details. For further information, the reader is encouraged to review the literature.