General factors of usability. Nielsen (2003) defines usability as follows: ‘…a quality attribute that assesses how easy user interfaces are to use’. He goes on to describe five quality components to define usability: learn-ability, efficiency, memorability, errors, and satisfaction. The learn-ability factor concerns how easy it is to learn to use the device. Efficiency determines the amount of resources to be invested in order to achieve the intended goal. The less the invested resource, e.g. time, the higher the efficiency. Efficiency also refers to the capability for matching functions provided by the product with the needs of the users to produce acceptable product performance without inducing frustration, fatigue, and dissatisfaction. The memorability component of usability indicates that the operation of a device should be easy to remember, thereby minimising the effort required to relearn how to use the device following periods of non-use. Errors are generally much more easily identifiable from products with computer interfaces, where error messages are signalled to the user. However, in a broader context errors can be construed as user actions that do not accomplish the desired goal. In any case, errors resulting from interacting with the product should be minimal, and if they do occur, the user should be able to easily recover from them. Finally, the satisfaction component addresses the experience the user has in interacting with the product, that is, the experience should be satisfying (Xxxx, 2004). The International Organisation of Standardisation sets out seven general ergonomic principles (DIN EN ISO 9241, part 10), which should be adopted in the design of any dialogue to make it more effective, efficient and satisfying to use.
