13 Research methods, statistics, critical review and evidence-based practice13.6 Economic analysis
13.6.7.5 Time trade-off
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Written by Resident Doctor Committee · Published
New in the 2027 syllabus
The College added this topic for exams from January 2027.
What it is
Time trade-off puts a number on a health state. This number is called a health-state utility. The method asks how many years in full health a person sees as equal to a longer time in the health state being valued.
The point where the person values both options equally is called indifference. In the simple version, both options end in death. The utility is then the full-health years divided by the years in the impaired state.
Worked example
A hypothetical respondent is indifferent between these two options.
Option | Health state | Duration |
|---|---|---|
A | A specified impaired health state | 10 years |
B | Full health | 7 years |
Give full health a utility of 1 and death a utility of 0.
Set the two options equal, without discounting: 10 x utility = 7 x 1.
So utility = 7 / 10 = 0.70.
Under this simple model, 10 years at utility 0.70 are worth 7 quality-adjusted life years.
Limits
Answers are shaped by time preferences and by attitudes to giving up life. The result is not a direct physiological measurement.
Some states are valued worse than death. These need an adapted way of asking and may have negative utilities. Do not apply the simple formula for positive states unchanged.
The framing of the question and the durations used can change answers. Different elicitation methods need not give the same utility.
Time trade-off varies how long a person lives, with that length of life certain. The standard gamble instead varies the probability of the outcomes.