Usability Engineering

of success or failure of any venture is the usefulness and usability of products and services it provide

The Decisive Factor

Ease of understandability
Speed of user task performance
User error rate
Retention over time
User satisfaction 
• Usability-in-the-large includes usefulness

Usability

Increased user and organization productivity

Decreased user training costs

Decreased user errors • A usable interface has a major impact on achieving these goals• Training is a costly substitute for usability• Ensuring a usable interface comes from usability engineering

Goals of Organizations

Product — the interface itself; content, human factors, look and feel, guidelines

Process — techniques and methods for interface development • A significant cause of poor usability in product is lack of understanding of proper development process 

Evolution of a good design requires...

Development of user interaction component
Interface behavior 
View of user & interaction developer

Development of user interface software component
Code instantiating interaction
View of system & programmer

User Interface Development

...is separate from, and fundamentally different from, user interface software development
• Different skills and abilities
• ISSUE: Interaction design is an iteratively evolving design spec for the interface software

User Interaction Development

• Developing the product
— the user interaction component
• Well-established guidelines, principles exist to help produce a more usable interface
Consistency 
Real-world metaphors
Short-term memory limits 
Message composition
Use of color
Screen layout
User locus of control 
Designing for user errors
Cognitive and physical affordances
Task thread continuity, etc...
• Well-established rules for use of interaction styles also exist
Menus 
Buttons 
Fields in a form, etc.

Key Issue in Usability

If you can't measure it, you can't manage it!
• Usability specifications — operationally defined, measurable goals, used as criteria for effective and efficient user interface
Based on:
- benchmark
tasks
- representative tasks
- mission critical tasks
* User performance (e.g., time, errors)
* User satisfaction (e.g., questionnaire)

Quantifying Usability