Usability diagnosis
Introduction
ISO 9126 was an international standard for software quality assessment. It was replaced in 2005 by the SQuaRE set of standards, ISO 25000:2014, which develops the same concepts.
Basics
This standard comes from the model established in 1977 by McCall and his colleagues, who proposed a model to specify software quality. The McCall quality model is organized around three types of Quality Characteristics:
• - Factors (specify): They describe the external view of the software, as seen by users.
• - Criteria (build): They describe the internal vision of the software, as seen by the developer.
• - Metrics (control): They are defined and used to provide a scale and method for measurement.
The standard provides an environment for organizations to define a quality model for the software product. Doing so, however, leaves each organization with the task of precisely specifying its own model. This could be done, for example, by specifying objectives for quality metrics which evaluate the degree of presence of quality attributes.
A software product is defined in a broad sense as: executables, source code, architectural descriptions, and so on. As a result, the notion of user is expanded to both operators and programmers, who are users of components such as software libraries.
The model includes internal and external metrics. Internal metrics are those that do not depend on the execution of the software (static measures), while external metrics are those applicable to the running software. Quality usage metrics are only available when the final product is used in real-world conditions. Ideally, internal quality does not necessarily imply external quality and this in turn means quality in use.
Characteristics
The standard is divided into four parts which address reality, external metrics, internal metrics and quality in usage and delivery metrics. The quality model established in the first part of the standard, ISO 9126-1"), classifies software quality into a structured set of characteristics and subcharacteristics. Each subcharacteristic (such as adaptability) is divided into attributes. An attribute is an entity which can be verified or measured in the software product. The attributes are not defined in the standard, since they vary between different software products. The characteristics are organized as follows: