Scematic illustration of the complementarity between QALYs and DALYs. QALYs are years of healthy life lived - DALYs are years of healthy life lost.

Whereas DALYs represent a loss and should be minimised, QALYs represent a gain and should be maximised.

In the DALY approach, the years are disability weighted on a scale from zero, which indicates perfect health (no disability), to one, which indicates death.

In the QALY approach, the scale goes the opposite way: A quality weighting (sometimes called "utility") of 1 indicates perfect health, whereas 0 indicates no quality of life, and is synonymous to death.

( Age weighting and discounting factors are not included in this illustration).