Quality requirement

A quality requirement is a specific, verifiable statement about a characteristic or attribute that a system or component must exhibit — how well it must perform, rather than what it must do. Runtime performance, safety, security, reliability, and maintainability are typical examples. Quality requirements are the concrete expression of non-functional requirements (NFR). Where an NFR states that a system shall be reliable, a quality requirement states how reliable, in measurable terms.

Each quality requirement targets one quality attribute, and should be phrased so it can be objectively tested. For example, "95% of requests return within 200ms" rather than "the system should be fast". A quality requirement left vague in this way is difficult to verify and easy to argue about after the fact.

Taken together, a system’s quality requirements define its target software quality, and they form the leaves that a quality tree organizes into a coherent hierarchy.