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# Quality-adjusted life years (QALYs)

- Measure

They combine how long people live and how well. A year in full health is worth 1 QALY; a year in poor health is worth less.

Updated Nov 5, 2026

## Key points

- QALY = years of life × utility of the health state (from 0 to 1).
- One year at utility 0.5 equals half a year in full health.
- Incremental cost-effectiveness ratio (ICER) = (cost B − cost A) / (QALY B − QALY A).
- QALYs are gained with an intervention (more is better); DALYs are lost to disease (more is worse).
- Utilities come from standardised quality-of-life questionnaires or from methods such as the standard gamble and the time trade-off.
- The cost per QALY that counts as acceptable is set by each country; it is not a fixed figure.

On this page

- The basics Key points
- What it is
- Formula and reading
- Worked example
- QALYs versus DALYs
- Markers
- How to read it
- Common pitfalls
- Memory aid

- Study Test yourself

- Connections Connections

## What it is

The QALY multiplies years of life by a utility weight from 0 (death) to 1 (full health). It lets interventions that lengthen life be compared with ones that improve it. In economic evaluation, the incremental cost-effectiveness ratio divides the extra cost of one option by the extra QALYs it produces.

## Formula and reading

- Total QALYs = Σ (years in each state × utility of that state).
- ICER = difference in costs / difference in QALYs; read as the cost of each extra QALY gained.
- If one option costs less and gives more QALYs, it dominates the other and no ratio is needed.
- Some very severe states can be valued below 0 (worse than death) in some methods.

## Worked example

- A made-up chronic condition has two approaches. A: 10 years at utility 0.6, costing 30 million pesos per patient.
- B: the same 10 years, at utility 0.8, costing 50 million pesos.
- QALYs for A: 10 × 0.6 = 6.0. QALYs for B: 10 × 0.8 = 8.0. Gain with B: 2.0 QALYs.
- Extra cost of B: 50 − 30 = 20 million pesos.
- ICER: 20 million / 2.0 QALYs = 10 million pesos per QALY gained.
- If B also extended life to 12 years at utility 0.8: 12 × 0.8 = 9.6 QALYs; gain 3.6 and ICER = 20 / 3.6 ≈ 5.6 million per QALY.
- Reading: B gives more health at an extra cost; whether it is adopted depends on the country's threshold and budget.

## QALYs versus DALYs

| Feature | QALY | DALY |
| --- | --- | --- |
| Question | How much healthy life does an intervention gain? | How much healthy life does a population lose to a disease? |
| Weight scale | Utility: 1 = full health, 0 = death | Disability: 0 = full health, 1 = equivalent to death |
| Direction | More is better | More is worse |
| Typical use | Economic evaluation of technologies | Burden of disease and priorities |

## Markers

- QALY
- Utility
- ICER
- Cost-utility

## How to read it

- Health technology assessment agencies use QALYs and cost-effectiveness ratios to recommend what to fund with public money.
- A trial that improves symptoms without changing survival can show a benefit in QALYs but not in mortality.

## Common pitfalls

- Dividing total cost by total QALYs of a single option: the decision rests on the incremental ratio.
- Confusing the utility scale (1 is good) with the disability scale (1 is bad).
- Forgetting that utilities depend on who values them (patients or the general public).

## Memory aid

Years times quality, like quantity times price: the QALY multiplies time by how well it is lived.

## Test yourself

Think of the answer, then tap the question to see it.

## How many QALYs are 4 years at utility 0.75?

4 × 0.75 = 3 QALYs.

## How is the incremental cost-effectiveness ratio calculated?

By dividing the difference in costs by the difference in QALYs between the two options being compared.

## Option B costs 12 million more and gives 3 more QALYs than A. What is the ICER?

12 / 3 = 4 million per QALY gained.

## What does it mean when one option dominates another?

It costs less and produces more QALYs; it is preferred without needing to calculate the ratio.

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## Connections

## Related 4

- [Disability-adjusted life years (DALYs)](https://durgical.com/en/epidemiology/disability-adjusted-life-years/)
- [Life expectancy](https://durgical.com/en/epidemiology/life-expectancy/)
- [Number needed to treat](https://durgical.com/en/epidemiology/number-needed-to-treat/)
- [Clinical practice guidelines](https://durgical.com/en/epidemiology/clinical-practice-guidelines/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Chapter 20: Economic evidence](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-20) (Cochrane, 2024); [Making Choices in Health: WHO Guide To Cost-Effectiveness Analysis](https://www.who.int/publications/i/item/9241546018) (World Health Organization, 2003).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/epidemiology/quality-adjusted-life-years/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/quality-adjusted-life-years/