The QALY combines these two benefits into one metric, using survey data about the trade-offs people are willing to make in order to assess how bad different sorts of illnesses or disabilities are. For example, on average people rate a life with untreated AIDS as 50% as good as life at full health; people on average rate life after a stroke as 75% as good as life at full health; and people on average rate life with moderate depression as only 30% as good as life in full health.
