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Usability Usability of a computer system is a combination of factors: • User interface design • Functionality • Performance • Help systems and documentation • Freedom from errors
Levels of usability interface design functional design data and metadata computer systems and networks conceptual model
Conceptual model The conceptual model is the user's internal model of what the system provides: • The desk top metaphor -- files and folders • The web model -- click on hyperlinks • Library models search and retrieve search, browse and retrieve
Hierarchical browsing Level 2 Level 1 Level 0
Interface design The interface design is the appearance on the screen and the actual manipulation by the user • Fonts, colors, logos, key board controls, menus, buttons • Mouse control or keyboard control? • Conventions (e.g., "back", "help") Example: Screen space utilization in American Memory page turner.
Principles of interface design Interface design is partly an art; there are general principles: • Consistency -- in appearance, controls, and function. • Feedback -- what is the computer system is doing? why does the user see certain results? • Users should be able to interrupt or reverse actions • Error handling should be simple and easy to comprehend • Skilled users offered shortcuts; beginners have simple, well-defined options The user should feel in control
Functional design The functional design, determines the functions that are offered to the user • Selection of parts of a digital object • Searching a list or sorting the results • Help information • Manipulation of objects on a screen • Pan or zoom
Same functions, different interface Example: the desk top metaphor • Mouse -- 1 button (Macintosh), 2 button (Windows) or 3 button (Unix) • Close button -- left of window (Macintosh) right of window (Windows)
Data and metadata Structural data and metadata stored by the computer system enable the functions and the interface • The desktop metaphor has the concept of associating a file with an application. This requires a file type to be stored with each file: -- extension to filename (Windows and Unix) -- resource fork (Macintosh)
Computer systems and networks The performance, reliability and predictability of computer systems and networks is crucial to usability • Response time instantaneous for mouse tracking and echo of key stroke 5 seconds for simple transactions • Example: Pipelined algorithm for the Mercury page turner • Quality of Service for real time information
Design tensions in networked systems • Client computers and network connections vary greatly in capacity • Client software may run on various operating systems; it may be current or an earlier version • System designers wish to control clients; users wish to configure their own environments
Varieties of user interfaces
End user interface. Allows a library user to search , browse , or retrieve known items. Uses structural metadata to present the collections to the user.
Librarian and system administrator interface. Provides services for an authenticated user to view , add , delete , or edit digital objects.
Batch interface. Provides a method to load large numbers of digital objects automatically.
Structure editor. Allows user to review and edit the structure of a collection.
Disabilities • What if the user: is visually impaired or color blind? does not speak English? is a poor typist? • There is a tradition of blind programmers • Navigation of web sites need not be only visual
Evaluation of usability • Observing users (user protocols) • Focus groups • Measurements effectiveness in carrying out tasks speed • Expert review • Competitive analysis
Usability and cost • Performance may be expensive in hardware or special software development • User interface development may be a major part of a software development project • Costs are multiplied if a user interface has to be used on different computers or migrate to different versions of systems Web browsers provide a general purpose user interface that others maintain