# On the Use of Quality Models to Address Distinct Quality Views
## 1 Introduction  
Software product quality models have undergone continuous development for more than 40 years, starting with their conception in the 1970s [1, 2, 3, 4]. Assessing software products involves quality models of a wide range from simple hierarchic decomposition techniques to complex meta-models to cope with the abstract notion of software quality [3, 4, 5]. Nevertheless, the quality models applied and their concrete implementations differ significantly in terms of the ability to capture the manifestations of software quality. The differences in this ability of the identified software product quality models are introduced in the present paper to assist with the decision of whether a particular quality model is suitable for a specific task or whether the statement based on a specific software product quality model holds for software quality in general. Following the terminology of the ISO/IEC 9126 standard and its successor ISO/IEC 25010 [6, 7], the quality manifestations the software product quality models are able to capture are called quality views in this paper, by which a unified terminology is also laid down. Creating a unified terminology is necessary, as some software product quality models, including ADEQUATE [8, 9],FURPS+ [10; 11; 12], GEQUAMO [13], reflect the opinions of different stakeholders, which are directly or indirectly referred to as quality views in the model definition documents. The ISO/IEC 9126 and ISO/IEC 25010 standards [6; 7] distinguish three distinct views: (1) internal quality, (2) external quality, and (3) quality-in-use view [6; 7; 14]:  
**Internal quality view:**: The set of quality properties of the source code and documentation that are deemed to influence the behaviour and use of the software. **External quality view:**: The set of quality properties that determine how the software product behaves while it operates. These properties are usually measured when the software product assessed is operational and is being tested. **Quality-in-use view:**: The set of quality properties that determine how the user perceives software product quality, i.e., to what extent the objectives that the software is used for can be achieved. The software product quality assessed by the three different views needs to have predictive power towards each other, called predictive validity [6; 7]. Thus, the measured internal quality should predict the external quality, and the measured external quality should predict the quality-in-use [6; 7]. Nevertheless, this validity does not necessarily hold since quality metrics and measures associated with the quality properties do not always have such a predictive power [14]. Software product quality model families explicitly or implicitly define which manifestations of quality they are able to address. The term software product quality model family is coined for related quality models with the same concepts but with minor differences. While the terms software product quality model family and software product quality framework are used interchangeably here, a distinction is made between the product quality model and process quality model. The former deals with the quality properties of software products while the latter assesses the processes through which the software products come into existence [6; 7]. Furthermore, the term software product quality model or quality model in short refers to any software product quality model or any software quality assessment approach that makes the use of a software product quality model possible. This research considers quality models that aim to describe each known aspect of software product quality, i.e., models that aim for completeness and, therefore, do not focus solely on a limited scope of quality properties such as maintainability or security. Consequently, the identified software product quality models offer support for the assessment of the software product as a whole and allow to define quality targets that endeavour for completeness.