Usability Engineering

1
Usability Engineering

• CptS 443
• Human-Computer Interaction

2
Usability Engineering

• The ultimate test of a products usability is
  based on measurements of users experience with
  it.

3
Usability Specification

• How do we describe usability?
• Part of the requirements specification
• Whereas req. spec. focuses on function
• Usability spec. focuses on attribute
• Feature of the interface
• Scope of each individual specification
• Used to gauge usability

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Usability Specification Components

• What is a usability specification?
• Attribute
• Usability principle
• Measuring concept
• Concrete example
• Measuring method
• Quantitative

• Current level
• Existing system
• Worst case
• Still acceptable
• Planned level
• Target
• Best case
• Feasible

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VCR Example

• Attribute backward recoverability
• Measuring concept Undo an erroneous programming
  sequence
• Measuring method of user actions
• Current level not supported
• Worst case less than forward recover
• Planned level two actions
• Best case one action

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Measuring Methods

• Time to complete
• task (/time)
• successes / failures
• time in error
• errors ( or )
• better ( or )
• steps
• Freq. of paper clip
• complaints
• repeated failures

• runs succ. or fail.
• misled
• good/bad features
• cmds ignored
• regressive acts
• users prefer
• workarounds
• disruptions
• losing control
• user frustrated

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Relative Measurement Levels

• Existing system
• Previous version
• Competitor
• No computer
• Absolute scale
• Prototype
• Earlier performance
• Component
• Variance

8
ISO 9241 Metrics
9
Problems

• Usability engineering satisfies usability
  specifications
• Difficult to determine proper measure
• Early in system design
• Empirical
• Subjective

10
Iterative Design

• How to overcome the specification problem
• Prototypes
• Disposable
• Incremental
• Evolutionary
• Problems
• time, planning
• Lack of complete features
• Contracts

11
Prototyping Techniques

• How to build a quick fake
• Storyboards
• Crippleware
• Language support
• Rapid prototyping

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Design Rationale

• Why is a system the way it is?
• Structural/architectural description
• Functional/behavioral description
• Benefits
• Communication, documentation
• Accumulated knowledge
• Formality forces forethought
• Investigation of alternatives
• Context

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Methods

• How do I form a design rationale?
• Process oriented design rationale
• Issue, positions, arguments
• Design space analysis
• QOC questions, options, criteria
• DRL decision, alternatives, goal
• Psychological design rationale
• task-artefact cycle
• scenarios