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 quantify the “usabilityusability” of a system - no

universal definition of the term

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

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

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

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