Six Styles for Usability Requirements

A system can have adequate functionality, but inadequate usability because it is too difficult to use. The purpose of usability requirements is to guard against that. This paper shows six styles for usability requirements seen in practice or recommended by experts. For each style we discuss how we can verify the requirements, how we can use them during development, how we elicit the data for the specification, and to what extent the style covers the essence of usability. Introduction The largest part of the requirements specification deals with the functional requirements, that is the system input, processing, and output. These requirements say nothing about how easy the system is to use, yet ease-of-use is a major concern with most systems. A usability requirement specifies how easy the system must be to use. Usability is a non-functional requirement, because in its essence it doesn't specify parts of the system functionality, only how that functionality is to be perceived by the user, for instance how easy it must be to learn and how efficient it must be for carrying out user tasks. Surprisingly, the literature has very little to say about usability requirements and rarely provides real-life examples. Nielsen (1993), Preece (1994, chapter 19), and Macaulay (1996) give much advise on usability requirements, but in a rather abstract setting without real-life examples. Also practitioners have great difficulties specifying usability requirements and often end up stating that "the system shall be easy to use". Every now and then, however, we have come across more meaningful and precise usability requirements. They have been important for this study. The usability requirements must be tangible so that we are able to verify them and trace them during the development. They must also be complete so that if we fulfill them, we are sure that we get the usability we intend. Meeting these goals is difficult in practice and no approach seems to cover all of them. Below we study six usability styles and discuss their strengths and weaknesses. In practice you can use the styles in combination. Usability Factors Before we look at the different styles, we will briefly discuss what usability is. According to a traditional definition, usability consists of five usability factors: 1. Ease of learning. The system must be easy to learn for both novices and users with experience from similar systems. 2. Task efficiency. The system must be efficient for the frequent user.