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ISO/IEC TR 9126-3:2003 provides internal metrics for measuring attributes of six external quality characteristics defined in ISO/IEC 9126-1. ISO/IEC TR 9126-2 defines external metrics, ISO/IEC TR 9126-3:2003 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 9126-3:2003 are not intended to be an exhaustive set. Developers, evaluators, quality managers, maintainers, suppliers, users and acquirers may select metrics from ISO/IEC TR 9126-3:2003 for defining requirements, evaluating software products, measuring quality aspects and other purposes.

Users of ISO/IEC TR 9126-3:2003 can select or modify and apply metrics and measures from ISO/IEC TR 9126-3:2003 or may define application-specific metrics for their individual application domain. For internal metrics view, there are pure internal metrics proposed for reference purposes.

ISO/IEC TR 9126-3:2003 is intended to be used together with ISO/IEC 9126-1.

ISO/IEC TR 9126-3:2003 contains:

• an explanation of how to apply software quality metrics;
• a basic set of metrics for each subcharacteristic;
• an example of how to apply metrics during the software product life cycle.

ISO/IEC TR 9126-3:2003 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.