Quality and usability: A new framework

ISO/IEC 9126 (1991) established a practical way of decomposing software quality into a set of characteristics and subcharacteristics. Reconciling this approach to quality with a new standard for usability (ISO 9241-11) has led to a comprehensive framework for software product quality which is being incorporated in a revision to ISO/IEC 9126. The new framework defines three perspectives: internal quality (static properties of the code), external quality (behaviour of the software when it is executed) and quality in use (whether the software meets the needs of the user when it is in use). Quality in use is a broader view of the concept of usability defined in ISO 9241-11. ISO/IEC 14598 describes a process for evaluating software product quality which is consistent with this model. Software quality characteristics: ISO/IEC 9126 In order to evaluate software it is necessary to select relevant quality characteristics. This can be done using a quality model which breaks software quality down into different characteristics. ISO/IEC 9126 (1991) provides a general-purpose model which defines six broad categories of software quality: functionality, reliability, usability, efficiency, maintainability and portability. These are further broken down into subcharacteristics which have measurable attributes (figure 1). The ISO/IEC 9126 characteristics and subcharacteristics provide a useful checklist of issues related to quality. The actual characteristics and subcharacteristics which are relevant in any particular situation will depend on the purpose of the evaluation, and should be identified by a quality requirements study.