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Usability What is usability Easy to use? User Friendly? A system with a GUI?
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Usability Engineering
No clear definition until fairly recently First proposed by Shakel 1987 Tried to provide an operational
definition of Usability quantify the “usabilityusability” of a system - no
universal definition of the term
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Usability Shackle
LEAF specifications take the form of precise
statements of performanceperformance goals. Typically related to
Ease of learning (LearnabilityLearnability) ThroughputThroughput (Efficiency) SubjectiveSubjective matters of user satisfaction (Attitude) Flexibility
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Usability goals Effective to use Efficient to use Safe to use Have good utility Easy to learn Easy to remember how to use
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Activity on usability How long should it take and how
long does it actually take to: use a VCR to play a video? use a VCR to pre-record two
programs? use an authoring tool to create a
website?
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User experience goals Satisfying - rewarding Fun - support creativity Enjoyable - emotionally fulfilling Entertaining …and more Helpful Motivating Aesthetically pleasing Motivating
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Usability and user experience goals How do usability goals differ from
user experience goals? Are there trade-offs between the
two kinds of goals? e.g. can a product be both fun and
safe? How easy is it to measure usability
versus user experience goals?
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Usability Evaluation Techniques
There are 4 core evaluation paradigms
Quick and Dirty Usability Testing Field Studies Heuristic or predictive evaluation
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Quick and Dirty Is common practice where designers get
feedback from users or HCI consultants to check there ideas are ok
Can be done at any stage – emphasis is on speed rather than detailed well documented findings – do you like this layout, this icon?
Is used when timescales are short Has become particularly popular in web
design
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Usability testing Dominant approach in 80’s remains
very important – typically lab based Move towards other forms including
field testing and heuristic evaluations Testing involves measuring typical
users performing real tasks and recording the time taken or number of errors made
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Usability – Field Tests Field studies of usability are done in the
users natural setting Aims to determine what the user
actually does and what other elements may impact on the interaction with the device or system
Field studies can be used to Identify opportunities for new products Determine requirements for design
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Usability testing Recording of users performance may be
through a number of techniques including Video recording Direct observation Software logging
Resulting data is used to inform the design and engineer it to meet the desired performance levels – usability engineering
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Usability principles/heuristics Similar to design principles, except
more prescriptive Used mainly as the basis for
evaluating systems Provide a framework for heuristic
evaluation Experts guided by heuristics to
predict usability problems
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Usability specifications
Whiteside et al proposed that usability could be measured in terms of attributes which can be measured
Important because they offer a way of incorporating usability into software quality assurance specifications
Example (of a data retrieval system) “90% of all users will be able to
identify the location of a book in the library by its shelf number, given the author’s name and book title, within ten seconds...”
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Objective of usability specifications To make decisions about user interface goals
explicit Support effective collaboration within the design
team, between its members Usability specification becomes part of the
general requirements specification Is an essential and integral part of UCD
Can be used to resolve conflict between,for example, users, analysts and designers over parts of the design
Can help determine whether need for further iterations and prototyping
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Usability Attributes Attributes should be measurable Example Attributes include
Time to complete a task % of task completed Number or % of errors made % of users who like the design No of times user asks for help/gets lost
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Components of a usability specification
Statement of the usability goal Statement of the usability attributeusability attribute Statement of the criteriacriteria which will represent
attainment of the specification Now level Worst case Planned level Best case
Statement of which set or subset of usersusers the specification applies to
A statement of preconditionspreconditions for the measurement
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Sample specification
Usability Goal: Users can quickly obtain a balance from an ATM Attribute: Time Measuring method: Observe and record
Time taken by users to obtain balance Now level: 30 secs Worst case: 25 secs Planned level: 20 secs Best case: 15 User class: All Preconditions: Users are familiar with ATMs
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Usability Specification Provide an alternative usability
specification using a different attribute that could determine whether the stated goal had been achieved
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Usability principles (Nielsen 2001)
Visibility of system status Match between system and the real world User control and freedom Consistency and standards Help users recognize, diagnose and recover from
errors Error prevention Recognition rather than recall Flexibility and efficiency of use Aesthetic and minimalist design Help and documentation
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Problems with UE Assertion of clear measurable criteria Measures specific user actions in
specific situations Other factors may contribute to overall
performance Setting appropriate criteria
Important to look for ‘now’ level
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