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User Interface Design and Ergonomics - National Open University of ...

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3.2 USABILITY CONCEPTS AND GUIDELINES<br />

The major concepts <strong>of</strong> usability are:-<br />

Efficiency: Once users have learned the design, how quickly can they perform tasks?<br />

Learnability: How easy is it for users to accomplish basic tasks the first time they<br />

encounter the design?<br />

Memorability: When users return to the design after a period <strong>of</strong> not using it, how easily<br />

can they re establish pr<strong>of</strong>iciency?<br />

Errors: How many errors do users make, how severe are these errors, <strong>and</strong> how easily can<br />

they recover from the errors?<br />

Satisfaction: How pleasant is it to use the design?<br />

Usability is <strong>of</strong>ten associated with the functionalities <strong>of</strong> the product, in addition to being<br />

solely a characteristic <strong>of</strong> the user interface (cf. framework <strong>of</strong> system acceptability, also<br />

below, which separates usefulness into utility <strong>and</strong> usability). For example, in the context<br />

<strong>of</strong> mainstream consumer products, an automobile lacking a reverse gear could be<br />

considered unusable according to the former view, <strong>and</strong> lacking in utility according to the<br />

latter view.<br />

When evaluating user interfaces for usability, the definition can be as simple as "the<br />

perception <strong>of</strong> a target user <strong>of</strong> the effectiveness (fit for purpose) <strong>and</strong> efficiency (work or<br />

time required to use) <strong>of</strong> the <strong>Interface</strong>". Each component may be measured subjectively<br />

against criteria e.g. Principles <strong>of</strong> <strong>User</strong> <strong>Interface</strong> <strong>Design</strong>, to provide a metric, <strong>of</strong>ten<br />

expressed as a percentage.<br />

It is important to distinguish between usability testing <strong>and</strong> usability engineering.<br />

Usability testing is the measurement <strong>of</strong> ease <strong>of</strong> use <strong>of</strong> a product or piece <strong>of</strong> s<strong>of</strong>tware. In<br />

contrast, Usability Engineering (UE) is the research <strong>and</strong> design process that ensures a<br />

product with good usability.<br />

Usability is an example <strong>of</strong> a non-functional requirement. As with other non-functional<br />

requirements, usability cannot be directly measured but must be quantified by means <strong>of</strong><br />

indirect measures or attributes such as, for example, the number <strong>of</strong> reported problems<br />

with ease-<strong>of</strong>-use <strong>of</strong> a system.<br />

The key principle for maximizing usability is to employ iterative design, which<br />

progressively refines the design through evaluation from the early stages <strong>of</strong> design. The<br />

evaluation steps enable the designers <strong>and</strong> developers to incorporate user <strong>and</strong> client<br />

feedback until the system reaches an acceptable level <strong>of</strong> usability.<br />

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