1 Tailoring the 9126 Quality Model Background As noted in ISO/IEC 9126-1, "Software quality should be evaluated using a defined quality model." The 9126-1 standard provides a hierarchical quality model, composed of characteristics and subcharacteristics, that we can tailor to suit our organizational quality goals and needs1. We can then use the tailored quality model as the foundation for the quality evaluation activities implemented as part of a Software Quality Assurance program. 9126-1 Quality Model Quality Model Framework ISO/IEC 9126-1 defines a quality model in terms of internal quality, external quality, and quality in use. Internal quality is evaluated using internal attributes of the software (design modularity and compliance with coding standards, for example). External quality is evaluated when the software is executed, typically during formal testing activities. Quality in use refers to the user’s view of the software quality when they use it in a particular environmental context. In other words, quality in use is evaluated after the software is deployed to the operational environment. These relationships are shown in the figure below2. Figure 1. Quality Category and Evaluation Relationships As 9126-1 observes, “Appropriate internal attributes of the software are a pre-requisite for achieving the required external behaviour, and appropriate external behaviour is a pre-requisite for achieving quality in use.” External quality and internal quality (and quality in use) are defined using attributes selected based on a given quality model. The two components of the 9126-1 quality model are discussed in the following subsections. 1 Other quality models, such as those proposed by Boehm and McCall, have been around since the late 1970's. ISO/IEC 9126-1 is a better choice as the basis for our tailoring both because it's an international standard and because it's much more recent. 2 All figures in this section are from ISO/IEC 9126-1.2 Figure 2. Quality in the Lifecycle External and Internal Quality ISO/IEC 9126-1 defines external and internal quality using six characteristics, which are further subdivided into subcharacteristics (see the figure below). Figure 3. External and Internal Quality Due to space limitations, we only define the six characteristics here; characteristic definitions are direct quotes from 9126-1. Additional detail is provided in the Appendix. Functionality The capability of the software product to provide functions which meet stated and implied needs when the software is used under specified conditions. Reliability The capability of the software product to maintain a specified level of performance when used under specified conditions.3 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. Quality in Use ISO/IEC 9126-1 defines quality in use using four characteristics (see the figure below). Figure 4. Quality in Use The definitions provided below are direct quotes from 9126-1. 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.4 Satisfaction The capability of the software product to satisfy users in a specified context of use. Tailoring the Quality Model All quality models, no matter what the source, should be tailored before being used by a particular organization; the 9126-1 quality model is no exception. By tailoring the model, we can focus on the quality characteristics and subcharacteristics that are important for to our particular organization. Tailoring can be accomplished at the organizational and/or application levels. It's typically best to provide a picture of the tailored model here, with detailed tailoring rationale for each characteristic and subcharacteristic provided in an Appendix. Conclusions We need to use a tailored quality model as the foundation for evaluation of software quality. As suggested in ISO/IEC 14598-6, the next step would be to document in detail how each of the quality characteristics will be evaluated; that's the topic of the next handout. References [1] ISO/IEC 9126-1:2001(E), Software Engineering – Product Quality – Part 1: Quality Model. [2] ISO/IEC 9126-4, Software Engineering – Product Quality – Part 4: Quality in Use Metrics. [3] ISO/IEC 14598-6, Software Engineering – Product Evaluation – Part 6: Documentation of Evaluation Modules.A-1 Appendix It's generally effective to include the detailed tailoring rationale for each of the subcharacteristics in the ISO/IEC 9126-1 quality model in an appendix like this one. Although the rationale can certainly be reviewed during more general discussions about the tailored model, it's also important that the appendix provides detailed historical support for the tailoring. Subcharacteristic definitions are direct quotes from 9126-1. Characteristic definitions from 9126-1 are provided for ease of reference. Characteristics and subcharacteristics are annotated to identify whether they’re included in the tailored model (“Yes”) or not (“No”). External and Internal Quality Functionality The capability of the software product to provide functions which meet stated and implied needs when the software is used under specified conditions. Included: Rationale: Suitability The capability of the software product to provide an appropriate set of functions for