Usability & Usability Engineering

1. Usability & Usability Engineering

2. Usability • What is usability • Easy to use? • User Friendly?

3. Usability Engineering • No clear definition until fairly recently • First proposed by Shakel 1987 • Tried to provide an operational definition of Usability • quantify the “usability” of a system - no universal definition of the term

4. Usability Shackle LEAF • specifications take the form of precise statements of performance goals. • Typically related to • Ease of learning (Learnability) • Throughput (Efficiency) • Subjective matters of user satisfaction (Attitude) • Flexibility

5. Usability specifications • Whiteside et al extended definition • Measured usability 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...”

6. 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

7. Components of a usability specification • Statement of the usability goal • Statement of the usability attribute • Statement of the criteria which will represent attainment of the specification • Now level • Worst case • Planned level • Best case • Statement of which set or subset of users the specification applies to • A statement of preconditions for the measurement

8. 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

9. 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

10. Usability Specification • Provide an alternative specification using a different attribute that could determine whether the stated goal had been achieved

11. 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

12. Usability Some Questions • What are the key factors for successful usability work? • Is usability recognised as an important design issue? • Are organisations designing for usability?

13. Further Reading • Preece Chpts 1, 6 & 7