ISO/IEC TR 9126-2:2003 provides external metrics for measuring attributes of six external quality characteristics defined in ISO/IEC 9126-1. ISO/IEC TR 9126-2:2003 defines external metrics, ISO/IEC TR 9126-3 defines internal metrics and ISO/IEC 9126-4 defines quality in use metrics, for measurement of the characteristics or the subcharacteristics. Internal metrics measure the software itself, external metrics measure the behaviour of the computer-based system that includes the software, and quality in use metrics measure the effects of using the software in a specific context of use.
The metrics listed in ISO/IEC TR 9126-2:2003 are not intended to be an exhaustive set. Developers, evaluators, quality managers and acquirers may select metrics from ISO/IEC TR 9126-2:2003 for defining requirements, evaluating software products, measuring quality aspects and other purposes.
Users of ISO/IEC TR 9126-2:2003 can select or modify and apply metrics and measures from ISO/IEC TR 9126-2:2003 or may define application-specific metrics for their individual application domain.
ISO/IEC TR 9126-2:2003 is intended to be used together with ISO/IEC 9126-1.
ISO/IEC TR 9126-2:2003 contains an explanation of how to apply software quality metrics, a basic set of metrics for each subcharacteristic and an example of how to apply metrics during the software product life cycle.
ISO/IEC TR 9126-2:2003 does not assign ranges of values of these metrics to rated levels or to grades of compliance, because these values are defined for each software product or a part of the software product, by its nature, depending on such factors as category of the software, integrity level and users' needs. Some attributes may have a desirable range of values, which does not depend on specific user needs but depends on generic factors; for example, human cognitive factors.