Usability requirements deal with how easy it is for an operator to make use of the system. According to Gould and Lewis, 1985, the notion of usability is that "any system designed for people to use should be easy to learn (and remember), useful, that contains functions people really need in their work, and b easy and pleasant to use". Usability requirements cannot be directed verified, since they involve subjective behaviors that often have to be collected over time.
Usability requirements are documented expectations and specifications designed to ensure that a product, service, process, or environment is easy to use. Requirements can be provided in a broad variety of formats by business units, customers, and subject matter experts. The following are illustrative examples of usability requirements.
Learnability: The time and effort required to reach a specified level of performance (also described as "ease of learning").
Throughput: The task accomplished by experienced users, the speed of task execution, and the errors made (also described as "ease of use").
Accessibility: Requirements that things be useful to as broad a group of people as possible including people with disabilities.
Credibility: The trust that your website engenders in your users also plays a part in the user experience. One of the biggest concerns users have online is the security (in many cases, they worry about privacy, too).
Naturally, the usability of a design is important. However, we need to consider usability alongside these other concerns to create a great user experience. The UX comes as much from graphical design, interactive design, content, etc. as it does from usability alone.
Usability requirements are gathered, along with functional and data requirements, using many of the same requirements gathering techniques such as interviewing and observation. The activity of gathering usability requirements is often known as usability study and it is closely allied to the evaluation process. Usability requirements are concerned with user satisfaction and the overall performance of the system. Determining usability requirements requires the analyst to undertake three main types of analysis. A task analysis is required to determine the characteristics, particularly the cognitive characteristics, required of the users by the system - such things as the search strategy required, the prerequisite knowledge and cognitive loading, and so on. Additionally, the analysts should conduct a user analysis. The purpose of this is to determine the scope of the user population who will utilize the system. This includes aspects such as required intellectual ability, cognitive salient characteristics of users that are viable across the user population.
Most user modeling relies on checklists of user characteristics. Finally, usability requirements demand an environmental analysis, which focuses attention on the environments within which the system is to operate. This should include aspects of the physical environment as well as aspects of the user support environment.
Usability requirements may be expressed in terms of performance measures, referred to as usability metrics, which are detailed in a usability specification. Examples of metrics used to assess performance can include completion time for specified tasks by a specified set of users, the number of errors per task, and the time spent on using documentation. The selection of the most appropriate metrics will depend on the type of system being tested. For example, with information retrieval systems search time may be a key design criterion, while for a general-purpose text editing system the users' feelings of security and enjoyment when using the system might be more important than performance metrics.
Relationship between requirements and usability
Requirements gathering involves the system analyst/designer in a wide range of activities aimed at eliciting a precise description of the functional, data, and usability requirements of the systems under construction. The analyst must be aware of the variety of techniques available to assist in this process - from paper-based checklists to diagrammatic techniques to prototyping, meetings, and walkthroughs. Many of these same methods are used in the evaluation. In fact, requirements gathering and evaluation are very closely related since it is essential to make sure that the requirements are properly understood by the designers. Prototyping systems, using mock-ups, games, scenarios, and experimental evaluation techniques also contribute to establishing user-centered, but functionally feasible and structurally meaningful system requirements. Inevitably there is much overlap between data and functional requirements on one hand and usability requirements on the other. The usability criteria specify constraints on the processing and are invaluable in ensuring that the system is designed with users in mind.