ISO-9126 | Quality Assurance

ISO-9126 offers a comprehensive framework to describe many attributes and properties we associate with quality. There is a strict hierarchy, where no sub-characteristics are shared among quality characteristics. However, certain product properties are linked to multiple quality characteristics or sub-characteristics. For example, various forms of redundancy affect both efficiency and maintainability.

Consequently, various alternative quality frameworks have been proposed to allow for more flexible relations among the different quality attributes or factors, and to facilitate a smooth transition from specific quality concerns to specific product properties and metrics.

ISO-9126 (ISO, 2001) provides a hierarchical framework for quality definition, organized into quality characteristics and sub-characteristics. There are six top-level quality characteristics, with each associated with its own exclusive (non-overlapping) sub-characteristics, as summarized below:

Functionality: A set of attributes that bear on the existence of a set of functions and their specified properties. The functions are those that satisfy stated or implied needs. The sub-characteristics include:

- Suitability
- Accuracy
- Interoperability
- Security

Reliability: A set of attributes that bear on the capability of software to maintain its level of performance under stated conditions for a stated period of time. The sub-characteristics include:

- Maturity
- Fault tolerance
- Recoverability

Usability: A set of attributes that bear on the effort needed for use, and on the individual assessment of such use, by a stated or implied set of users. The sub characteristics include:

- Understandability
- Learnability
- Operability

Efficiency: A set of attributes that bear on the relationship between the level of performance of the software and the amount of resources used, under stated conditions. The sub-characteristics include:

- Time behavior
- Resource behavior

Maintainability: A set of attributes that bear on the effort needed to make specified modifications. The sub-characteristics include:

- Analyzability
- Changeability
- Stability
- Testability

Portability: A set of attributes that bear on the ability of software to be transferred from one environment to another. The sub-characteristics include:

- Adaptability
- Installability
- Conformance
- Replaceability

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