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Usability Specifications. Howell Istance School of Computing (chapter 3.6 : McCracken and Wolfe ). Acknowledgements. The majority of slides have been produced by Daniel D. McCracken (City College of New York) and

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Usability specifications l.jpg

Usability Specifications

Howell Istance

School of Computing

(chapter 3.6 : McCracken and Wolfe )


Acknowledgements l.jpg
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.


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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


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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


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Preference measures

  • Often obtained using a Likert Scale


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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


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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


Example of a usability specification l.jpg
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’


Example of a usability specification9 l.jpg
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


Example of a usability specification10 l.jpg
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


Example of a usability specification11 l.jpg
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


Example of a usability specification12 l.jpg
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


Example of a usability specification13 l.jpg
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


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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


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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


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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


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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


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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


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‘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’