Our SQM Approaches » ISO 9126

According to our experience, the most detailed description of quality attributes and metrics of a software product can be found in the ISO 9126 standard which consists of the following parts:

  • Part 1: Quality model (ISO/IEC 9126-1:2001)
  • Part 2: External metrics (ISO/IEC TR 9126-2:2003)
  • Part 3: Internal metrics (ISO/IEC TR 9126-3:2003)
  • Part 4: Quality in use metrics (ISO/IEC TR 9126-4:2004

Part 1 describes the quality model applied for software production. The model consists of two parts: internal quality + external quality, and quality in use.

The first part of the model defines six characteristics for internal and external quality, which are further subdivided into subcharacteristics, that can be measured by internal metrics or by external metrics. (Internal quality attributes are attributes which characterize the software itself, for example software size. External attributes are depending on the environment in which the software is used, they cannot be measured on the software itself, only on the working software, for example: the number of errors).

The standard-family defines the following six characteristics (high-level software quality attributes) for internal and external quality:

Quality Attribute

Definition
Functionality

The capability of the software product to provide functions which meet stated and implied needs when the software is used under specified conditions. The functions satisfy the formulated or supposed conditions.

Reliability

The capability of the software product to maintain a specified level of performance when used under specified conditions.

Usability

The capability of the software product to be understood, learned, used and attractive to the user, when used under specified conditions.

Efficiency

The capability of the software product to provide appropriate performance, relative to the amount of resources used, under stated conditions.

Maintainability

The capability of the software product to be modified. Modifications may include corrections, improvements or adaptation of the software to changes in environment, and in requirements and functional specifications.

Portability

The capability of the software product to be transferred from one environment to another.

The combined effect for the user of the previously introduced six quality characteristics can be described by “quality in use” characteristics which are specified in the second part of the model (and detailed in Part 4 of the standard family). This part of the model deals with characteristics which can be observed from the user’s point of view when the software product is used under specified conditions. The standard-family specifies the following four “quality in use” characteristics (high-level software quality attributes):

Quality Attribute

Definition

Effectiveness

The capability of the software product to enable users to achieve specified goals with accuracy and completeness in a specified context of use.

Productivity

The capability of the software product to enable users to expend appropriate amounts of resources in relation to the effectiveness achieved in a specified context of use.

Safety

The capability of the software product to achieve acceptable levels of risk of harm to people, business, software, property or the environment in a specified context of use.

Satisfaction

The capability of the software product to satisfy users in a specified context of use.

For more information see: ISO/IEC 9126-1: Software engineering - Product quality - Part 1: Quality model (2001). ISO/IEC TR 9126-2: Software engineering - Product quality - Part 2: External metrics (2003). ISO/IEC TR 9126-3: Software engineering - Product quality - Part 3: Internal metrics (2003). ISO/IEC TR 9126-4: Software engineering - Product quality - Part 4: Quality in use metrics (2004)