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A QALY is a measure of the value of health outcomes that combines the value of length of life and quality of life (utility value) into a single number.
It assumes that a year of life lived in perfect health is worth 1 QALY (1 year of life × a utility value of 1 = 1 QALY), and that a year of life lived in a state of less than this perfect health is worth less than 1.
In order to determine the exact QALY value, it is sufficient to multiply the utility value associated with a given state of health by the years lived in that state.
QALYs are expressed in terms of "years lived in perfect health".
For example, ½ year lived in perfect health is equivalent to 0.5 QALYs (0.5 years × a utility value of 1), the same as 1 year of life lived in a situation with a utility value 0.5 (e.g. being bedridden) (1 year × a utility value of 0.5).
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