Usability and usability requirements

Usability is concerned with 'optimizing the interactions people have with interactive products' and is achieved by defining usability and user experience goals. These goals aim to promote products which are effective, easy to learn and enjoyable to use. (Preece et al. 2002:14) Usability requirements capture usability goals and related measurements of these goals. (ibid. 2002:207) One approach is 'usability engineering' which is a 'process whereby the usability of a product is specified quantitatively, and in advance'. (Tyldesley 1990:284) This means documenting planned levels of usability. For example with regard to the usability goal of 'efficiency' outlined below, one benchmark is the time taken to locate a specific entry in an alphabetical index of a product. This is a measurable usability goal. Other usability goals are more difficult to measure and user experience goals, which are discussed below, are subjective and so cannot be measured in this way. (Preece et al. 2002:182) In this case qualitative or 'subjective, attitudinal measures of usability' are used. (Tyldesley 1990:288) For example, interviews or surveys through questionnaires can be implemented. Once usability requirements have been defined, the product is developed iteratively until it is demonstrated that it achieves the projected levels of usability. (Preece et al. 2002:285) It is important that the product is easy to use, as the target users may have limited computer abilities. This can be achieved by defining usability requirements through consideration of the usability goals below. These requirements can also be considered in conjunction with design and usability principles such as visibility, consistency, constraints and affordance. (ibid. 2002:21-25) Furthermore, reference to design principle standards such as the International Organization for Standardization's ISO 14915 'Software ergonomics for multimedia user interfaces' provides a framework to assist the designer in standardisation.

Effectiveness can be described as the level of performance of completion of tasks, (Shackel 1990:32) and in more general terms, refers to 'how good a system is at doing what it is supposed to do'. (Preece et al. 2002:14)

Learnability relates to the time taken to learn a system. (Shackel 1990:32) According to Preece et al. a 'key concern is determining how much time users are prepared to spend learning a system'. (Preece et al. 2002:17) Here, the 'Ten-Minute Rule' can be applied, that is to say that a novice user can learn the system within ten minutes. (ibid. 2002:16)

Flexibility is a measure of how well the system can adapt to 'variation in tasks and/or environments beyond those first specified' and an example in a product might be an option to mute the sound, which may be useful if used in public environments. (Shackel 1990:32)

Efficiency concerns the way users are supported whilst carrying out tasks. For example, an interface can be considered as efficient if common tasks can be completed through a limited number of steps. An example is to utilise one button to access a bookmark index. (Preece et al. 2002:14)

Safety is concerned with protecting a user from undesirable conditions. In this case it refers to avoidance of errors. (Preece et al. 2002:15) For example a 'quit' function could be the standard Windows 'close' box in the top right hand corner, and as such is distant from other commonly used functions preventing accidental closure of the program. Another example is in providing a confirmation dialogue box before allowing deletion of bookmarks.

Utility is a goal relating to the 'extent to which the system provides the right kind of functionality', so that a user can achieve their goals. (Preece et al. 2002:16) One example might be an alphabetical search function which allows searching by word entry or by scrolling through a list.

Memorability refers to 'how easy a system is to remember how to use, once learned'. (Preece et al. 2002:17) This can be achieved by using meaningful icons and systematic placement of them. Furthermore, options can be structured in a logical way such that knowledge from the 'real' world can aid the user. An example is that information about Chile can be accessed through a 'South America' link.

Usability goals are coupled with user experience goals. These are concerned with 'what the interaction with the system feels like to the users' and focus on systems that are fun, satisfying, enjoyable, entertaining, helpful, motivating, aesthetically pleasing, emotionally fulfilling, rewarding and supportive of creativity. (Preece et al. 2002:18) This coupling leads to trade-offs, that is to say that the outcomes of the goals do not necessarily coincide. For example, engagement with the product may be encouraged by peripheral functions not being easy to use, raising a challenge to the user. This contradicts the usability goal of 'learnability'. (ibid. 2002:19)

About the Author

Mark Chambers does freelance web design in Surrey and focuses on affordable web design, search engine optimisation, content management systems and ecommerce.

Freelance Web Design Surrey

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