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Time trade-off (TTO) is one of a series of score-scaling techniques devised by economists in an attempt to assign a single numerical value to a health condition. The TTO values are calculated as a ratio of life expectancy in their current health state to an equivalent number of years at perfect health:
These values are known as health state scores or utility ratings, and are a global measure of health-related quality of life with a score on a scale ranging from 0 to 1.0. Values of 0 represent an extremely poor quality of life in those individuals for whom death would be equivalent to their present state. A score of 1.0 represents a very high quality of life in those individuals whose current health would be equivalent to their perception of perfect or full health.
For example, a 60 year old woman whose life expectancy may be a further 25 years, may report that 10 years of perfect health is equivalent to 25 years of life in her present condition(i.e., she would be willing to give up as many as 15 years of current life to achieve 10 years of perfect health.) The utility rating of their current health is:
10 years ÷ 25 years = 0.4
Increases in TTO values reflect a net gain in health-related quality of life.
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