Usability & Usability Engineering

Usability What is usability Easy to use? User Friendly?

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

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

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...”

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

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

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

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

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

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

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?

Further Reading Preece Chpts 1, 6 & 7