The term usability is often used to refer to the capability of a product to be used easily. This corresponds with the definition of usability as a software quality in ISO/IEC 9126:
"a set of attributes of software which bear on the effort needed for use and on the individual assessment of such use by a stated or implied set of users".
However the attributes which a product requires for usability depend on the nature of the user, task and environment. A product has no intrinsic usability, only a capability to be used in a particular context. Usability cannot be assessed by studying a product in isolation.
There are therefore three potential ways in which the usability of a product could be measured.
1. By analysis of the product and the context of use.
Usability could be measured by assessing the product features required for usability in a particular context. Appropriate features are specified in other parts of ISO 9241. However ISO 9241 only gives partial guidance. Of the many potential design solutions compatible with ISO 9241, some will be more usable than others.
2. By analysis of the process of interaction.
Usability could be measured by modelling the interaction between a user carrying out a task with a product. However, current analytic approaches do not give very precise estimates of usability. As the interaction is a dynamic process in the human brain, it cannot be studied directly. However, measures of mental effort and acceptability are important indirect measures of factors contributing to usability.
3. By analysing the effectiveness and efficiency which results from use of the product in a particular context and measuring the satisfaction of the users of the product. As these are direct measures of usability, they are the ultimate test of usability. If a product is more usable in a particular context, usability measures will be better.
It should be noted that usability as defined in this part of ISO 9241 also depends on software qualities which are distinct from usability as defined in ISO 9126, such as functionality, reliability and computer efficiency. These software qualities all contribute to quality of the work system in use.
Usability defined in terms of the quality of a work system in use necessarily depends on all factors which may influence use of a product in the real world, including organisational factors such as working practices and the location or appearance of a product, and individual differences between users including those due to cultural factors and prejudice. This broad approach has the advantage that it concentrates on the real purpose of design of a product - that it meets the needs of real users carrying out real tasks in a real technical, physical and organisational environment. This is consistent with the objectives of ISO 9241 described in ISO 9241-1.
D.2 Use in conjunction with other parts of ISO 9241
D.2.1 Relationship of context of use to usability
Any component of the context of use (user, equipment, task or environment) may be manipulated in order to change the usability of a product. The user interface may be improved by conforming to good dialogue design practices (e.g. ISO 9241 parts 10, and 12-17). In addition the fit between the user and the rest of the context of use may be improved through means such as selection and training. The task may be designed appropriately (e.g. ISO 9241 part 2). Aspects of the working environment such as lighting, noise, workstation design might be improved (e.g. ISO 9241 parts 3-9). However, when evaluating the usability of a product, the focus is on optimising the product for a given context.
A software designer needs to identify the anticipated users, tasks and environments using this part of ISO 9241, before designing appropriate usability attributes into the software following the guidance and requirements of ISO 9241 parts 10, and 12-17. However, this alone will not guarantee that a product reaches a required level of usability, as ISO 9241 does not provide an exhaustive specification of how to apply the general principles that make a product usable.
D.2.2 Use in conjunction with ISO 9241-10: Dialogue Principles
D.2.2.1 Relevance of the context of use
The way in which each dialogue principle in ISO 9241-10 should be applied to the design or evaluation of a product will depend on the context of use. It is necessary to identify the relevant characteristics of the intended users, tasks and environments before applying the dialogue principles for design or evaluation. Although all aspects of the context of use should be considered for every dialogue principle, "suitability for the task" deals with design issues which are closely associated with the task characteristics. When applying this principle particular consideration should be given to those tasks which particular types of user may need to perform to meet the goals of the user organisation. "Suitability for learning", "suitability for individualisation", and "conformity with user expectations" deal with design issues which are closely associated with the user characteristics. When applying these principles particular consideration should be given to the needs of different types of intended users when performing intended tasks in particular situations.
D.2.2.2 Suitability for learning and individualisation
The dialogue principle "suitability for learning" refers to the attributes of a product which facilitate learning. Actual learnability in a specific context can be measured by comparing the usability of a product for one user over time, or comparing the usability of a product for an experienced and inexperienced user.
The dialogue principle "suitability for individualisation" refers to attributes of the product which facilitate adaptation to the user's needs for a given task. Actual flexibility of use by different users for different tasks can be assessed by measuring the usability in a number of different contexts.
D.2.2.3 Use of dialogue principles as usability objectives
The dialogue principles are stated in terms of characteristics of software. Some of the principles are related to usability objectives which are desired properties of the product, e.g. achievement of goals, error tolerance and learnability (see Annex B).