# Testability

**In short:** Testability is how well a system supports its own testing. High testability makes faults easier to find by testing. Low testability means more test effort.

## Explanation

Testability is the degree to which a system, or a part of it under test, supports its own testing. In other words, the system helps you test it.

When testability is high, finding faults by testing is easier. A fault is a defect in the software (see Fault). When testability is low, the team needs more test effort.

**Observability.** Observability is a core part of testability. It means you can see test results, both intermediate and final. If a tester cannot see what the system did, they cannot judge whether it worked.

**Standard definition.** The ISO/IEC 25010 quality model (see ISO/IEC 25010 Product Quality Model) describes testability as how effectively and efficiently you can establish test criteria for a system, product or component.

**Where it sits.** ISO 25010 places testability under maintainability. Maintainability is about how effectively and efficiently you can modify a product. The arc42 quality model notes this placement. It argues that testability also belongs under reliability, so it should appear in both places.

**Why it matters.** When you maintain or change a system, testability reduces risk.

## Example

A team builds an order system. Testers can see the intermediate results of a price calculation, not only the final total. When a total is wrong, they see which step failed. In a system that shows only the final total, the same fault takes longer to find. That system has lower testability and needs more test effort.



## Checks

- The document states how testers can observe intermediate and final results of the system.
- Testability is listed as a quality goal where changes to the system are expected, with its link to risk reduction.
- Testability is treated under maintainability and also considered under reliability.

## Related

- [Testability General Scenario](https://arc.fuet.ch/concepts/quality-attributes/testability-general-scenario.md)
- [Maintainability](https://arc.fuet.ch/concepts/quality-attributes/maintainability.md)
- [Integrability, Deployability, Testability and Reliability Tactics](https://arc.fuet.ch/concepts/tactics/integrability-deployability-and-testability-tactics.md)
- [ISO/IEC 25010 Product Quality Model](https://arc.fuet.ch/concepts/quality-attributes/iso-iec-25010-quality-model.md)
- [Reliability](https://arc.fuet.ch/concepts/quality-attributes/reliability.md)
- [Fault](https://arc.fuet.ch/concepts/quality-attributes/fault.md)
- [Quality Attribute](https://arc.fuet.ch/concepts/quality-attributes/quality-attribute.md)
- [Architectural Tactic](https://arc.fuet.ch/concepts/tactics/architectural-tactic.md)

## Sources

- [[1801.02201] A survey on software testability](https://arxiv.org/abs/1801.02201) — arxiv.org, accessed 2026-11-09
- [Software testability - Wikipedia](https://en.wikipedia.org/wiki/Software_testability) — en.wikipedia.org, accessed 2026-11-09
- [Maintainability](https://iso25000.com/index.php/en/iso-25000-standards/iso-25010/57-maintainability) — iso25000.com, accessed 2026-11-08
- [Shortcomings of ISO 25010 | arc42 Quality Model](https://quality.arc42.org/articles/iso-25010-shortcomings) — quality.arc42.org, accessed 2026-11-09