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