Usability Specifications - PowerPoint PPT Presentation

1 / 19
About This Presentation
Title:

Usability Specifications

Description:

... the Back Button is used, indicating that user cannot find desired information ... One or more experts in the use of the system are given the same task and the ... – PowerPoint PPT presentation

Number of Views:15
Avg rating:3.0/5.0
Slides: 20
Provided by: howelli
Category:

less

Transcript and Presenter's Notes

Title: Usability Specifications


1
Usability Specifications
  • Howell Istance
  • School of Computing
  • (chapter 3.6 McCracken and Wolfe )

2
Acknowledgements
  • The majority of slides have been produced by
  • Daniel D. McCracken (City College of New York)
    and
  • Rosalee J. Wolfe (DePaul University) to accompany
    their book User-Centered Website Development
    (Pearson Prentice Hall, 2003)
  • These have been edited to provide a European
    context for the examples used.

3
Usability Specifications
  • A system specification covering usability of the
    form the system shall be easy to use is of
    little use
  • What easy to use means is open to many
    different interpretations (the client may have a
    very idea of this from the software or site
    developer)
  • A specification which can not be tested is also
    of little use
  • We need a measurable definition of usability
    based on
  • Performance measures
  • Preference (or subjective) measures

4
Performance measures a sampling
  • Time to locate a book at the Barnes Noble
    website
  • Time to fill in customer information and place
    order
  • Number of times the Back Button is used,
    indicating that user cannot find desired
    information
  • Number of clicks to find the time of a TV show
  • Percentage of tasks completed correctly
  • Number of calls to support line

5
Preference measures
  • Often obtained using a Likert Scale

Strongly disagree Disagree Neutral Agree Strongly agree
It was easy to find what I wanted
It was simple to choose size and color
I could pay for my purchase quickly
6
Definition - ISO9241 / 11
  • Usability - degree to which specified users can
    achieve specified goals in a particular
    environment with effectiveness, efficiency and
    satisfaction
  • effectiveness - measures of accuracy and
    completeness of the goals achieved
  • efficiency - measures of resources (e.g time,
    money, effort) used to achieve goals
  • satisfaction - measures of the physical comfort
    and subjective acceptability of the product to
    its users

7
Components of a usability specification
  • statement of the usability attribute
  • statement of how it will be measured
  • statement of criteria that will represent
    attainment of the specification
  • statement of the subset of users to which the
    specification applies
  • statement of pre-conditions of measurement (e.g
    period of training

8
Example of a usability specification
  • In the context of a library information system
    e.g.
  • Efficiency of first time use measured by
    retrieval time of specified information
  • 90 of student users without any prior training
    should be able to retrieve the shelf number of a
    book, given the author and the title within 10
    times the length of time an expert user would
    take to complete the same task

9
Example of a usability specification
  • In the context of a library information system
    e.g.
  • Efficiency of first time use measured by
    retrieval time of specified information
  • 90 of student users without any prior training
    should be able to retrieve the shelf number of a
    book, given the author and the title within 10
    times the length of time an expert user would
    take to complete the same task

Statement of usability attribute
10
Example of a usability specification
  • In the context of a library information system
    e.g.
  • Efficiency of first time use measured by
    retrieval time of specified information
  • 90 of student users without any prior training
    should be able to retrieve the shelf number of a
    book, given the author and the title within 10
    times the length of time an expert user would
    take to complete the same task

Statement of how it will be measured
11
Example of a usability specification
  • In the context of a library information system
    e.g.
  • Efficiency of first time use measured by
    retrieval time of specified information
  • 90 of student users without any prior training
    should be able to retrieve the shelf number of a
    book, given the author and the title within 10
    times the length of time an expert user would
    take to complete the same task

statement of the subset of users to which the
specification applies
12
Example of a usability specification
  • In the context of a library information system
    e.g.
  • Efficiency of first time use measured by
    retrieval time of specified information
  • 90 of student users without any prior training
    should be able to retrieve the shelf number of a
    book, given the author and the title within 10
    times the length of time an expert user would
    take to complete the same task

statement of criteria that will represent
attainment of the specification
13
Example of a usability specification
  • In the context of a library information system
    e.g.
  • Efficiency of first time use measured by
    retrieval time of specified information
  • 90 of student users without any prior training
    should be able to retrieve the shelf number of a
    book, given the author and the title within 10
    times the length of time an expert user would
    take to complete the same task

statement of pre-conditions of measurement
14
Testing specifications
  • Using a prototype, a number of people can be
    asked to complete the task and the following can
    be measured
  • Whether a person can find the information or not
  • The length of time it takes
  • One or more experts in the use of the system are
    given the same task and the time taken from them
    to complete the task is measured
  • If in a sample of 20 students, 18 or more
    complete the test task successfully within the
    specified time (ie within 10 times as long as an
    expert would take) then the specification has
    been achieved

15
Usability Engineering
  • extends the principles underlying usability
    specifications
  • provides techniques to direct resources to
    improve the system with respect to individual
    usability attributes
  • defines usability goals through metrics
  • sets planned levels of usability that have to be
    achieved
  • analyses the impact of possible design solutions
  • incorporates user defined feedback in product
    design
  • iterating through the 'design-evaluate-design'
    loop until planned levels are achieved

16
Impact Analysis
  • Partition total time task time error time
  • error time represents potential saving if time
    spent in error can be removed by designing out
    the cause of the error
  • determine those errors which contribute the most
    to error time component
  • allocating resources to designing out individual
    errors gives then a known saving in task
    completion time

17
Strengths of Usability Engineering
  • agreeing on a definition of usability
  • setting this definition in terms of metrics and
    usability goals
  • putting usability on a par with other usability
    goals
  • providing a method for prioritising usability
    problems

18
and Weaknesses.
  • assumption that usability that usability can be
    operationalised
  • requirement that practitioner is familiar with
    laboratory methods
  • cost of conducting usability tests
  • unnaturalness of testing environment

19
Laboratory vs. Real World
  • work context
  • e.g. lab. test - 6 pages
  • normal work - reports of over 30 pages
  • time context
  • lab test - time when task will be completed is
    prescribed
  • real world - individual has some control over
    scheduling
  • motivational context
  • clear differences between lab and real world
  • social context
  • lab test - no support
  • real world - social network of support - ask my
    friends
Write a Comment
User Comments (0)
About PowerShow.com