Wikipedia
Time-Trade-Off (TTO) is a tool used in health economics to help determine the quality of life of a patient or group. The individual will be presented with a set of directions such as:
Imagine that you are told that you have 10 years left to live. In connection with this you are also told that you can choose to live these 10 years in your current health state or that you can choose to give up some life years to live for a shorter period in full health. Indicate with a cross on the line the number of years in full health that you think is of equal value to 10 years in your current health state.
The line usually ranges from 1 to 10 and the person's score is calculated by dividing the number corresponding to their cross by 10. For example, if someone marks a cross at 4 on the TTO line, they would be given a TTO score of 0.4. This number is often used in turn to calculate quality-adjusted life years or QALYs. In our example if this person were to live for 4 years at their current health state this would be equal to 1.6 QALYs (4 * .4). QALYs enable health care decision makers to combine mortality and morbidity into a single interval scale.
Other tools that are used to determine health utility are the visual analogue scale (VAS), the standard gamble method, the EQ-5D, the Health Utilities Index (HUI), etc.