Title: Usability Specifications
1Usability Specifications
- Howell Istance
- School of Computing
- (chapter 3.6 McCracken and Wolfe )
2Acknowledgements
- 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.
3Usability 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
4Performance 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
5Preference 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
6Definition - 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
7Components 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
8Example 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
9Example 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
10Example 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
11Example 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
12Example 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
13Example 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
14Testing 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
15Usability 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
16Impact 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
17Strengths 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
18and 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
19Laboratory 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