Usability Specifications

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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.

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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

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
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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

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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

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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
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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
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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
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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
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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