HumanComputer Interaction

1
Human-Computer Interaction

• Usability Specifications

2
Lecture Overview

• Definition and purpose
• Usability factors
• Measuring instruments
• Usability specification process
• Setting usability levels
• Advantages and disadvantages

3
USABILITY

• Previously defined in terms of
• Learnability
• Throughput
• Flexibility
• Attitude

4
ISO 9241 Usability Definition (1990)

• The effectiveness,
• efficiency and
• satisfaction with which
• specified users can achieve
• specified goals in a
• particular environment.

5
Functional and Usability Specifications

• Functional specifications are central to ensuring
  system functionality
• Usability specifications are central to ensuring
  system usability

6
Specification Examples

• Example of part of a Functional Specification
• System must be able to calculate the gross and
  net pay, tax, and other deductions for every
  employee, given the hourly rate, hours worked,
  tax code, etc.
• Example of part of a Usability Specification
• User must be able to enter employee
  information, for X employees, inside a three-hour
  period on a Friday morning

7
Purpose of Setting Usability Specifications

• Establish when an interface is good enough
• When to stop iteration

8
Common Usability Factors

• Speed of operation
• Completion rate
• Error free rate
• Satisfaction rating
• Learnability
• Retainability
• Advanced feature usage

Main software usability measures identified by
IBM
9
Speed of Operation

• A measure of the
• delay between
• initiating an action and
• achieving the users goal.

10
Completion Rate

• Given a set of key (benchmark) tasks, how many
  of them can the average user actually do, or
  complete, maybe within a specified time.

11
Error-free Rate

• The percentage of user actions that did not
  result in an error being made.
• Or
• In using the software for a specified time, what
  proportion of the time was not spent in dealing
  with errors?

12
Satisfaction Rating

• A subjective judgement by the user.
• Can be quasi-quantified using a rating scale.
• Very important for user acceptance.

13
Learnability

• Difficult to quantify
• May take long time to judge

14
Retainability

• By definition, requires time to test
• Could use benchmark tasks, performed at intervals
  (weeks, months, etc.)

15
Advanced feature usage

• 80 of users only ever use about 20 of the
  features available in a package.
• Sometimes an advanced feature is used in a
  simple way, not intended by designers (e.g.
  3-dimensional spreadsheet feature used as paper
  clip)

16
Time Dimension for Usability Factors

• Initial performance
• Long-term performance
• First impression
• Long-term user satisfaction

17
Measuring Instrument

• Method for providing values for a particular
  usability factor in a consistent manner
• Always quantitative (i.e. numerically measured)
• Objective
• Observable user performance
• Often associated with a benchmark test - involves
  typical (normally simple and frequent) task a
  user will perform
• Subjective
• User opinion
• Usually associated with a user questionnaire
• Objective and subjective measures are equally
  important

18
Usability Specification Process

• Defining usability through metrics
• Duration metrics
• Count measures
• Proportion completed
• Quality
• Setting and agreeing planned levels of metrics
• Analysing impact of alternative design solutions
• Incorporating user-derived feedback
• Iterating until planned levels are achieved

19
Sample Rows from a Usability Specification Table
for DTP Package
Task Issue Value to be Current Worst Planned
Best Measured Level Acceptable
Target Possible Level Level Leve
l Install- Installation Length of Many 90
mins 30 mins 20 mins ation task per time to
cant
benchmark successfully
install number 1 install Initial Se
t a tab Number of 3 errors 3 errors 2 errors 0
errors Performance errors Initial Delete a
tab Length of 6 secs 6 secs 4 secs 2 sec
Performance time on first
trial First Questionnaire Average score
?? 3 4.2 5 Impression (range 1-5)
20
Example Benchmark Tasks Electronic Mail

• Set up parameters for two terminals (ADM3/VT100)
• Set up auto-dial and auto-login
• Initiate dial-up and login
• Start on-line usage
• Send and receive a file
• Set slave printer on/off
• Set up present unattended dial up/login/file
  transfer
• Read received file
• Leave the system

Note Each task would be specified in precise
detail to ensure all participants attempt exactly
the same task
21
Setting Levels for Usability

• Restrict to major/critical issues
• Method Informed Guesses made by a team
• E.g. user interaction designer, user interface
  software developer, marketing person, project
  manager
• Generates team commitment to usability targets
• How will tasks be assessed?/what level of
  performance are expected?
• Initial performance and first impression are
  almost always appropriate

22
Current and Worst Acceptable Levels

• Current Level
• Present level of value to be measured not known
  at outset
• Possible even from manual system or initial
  prototype
• May be related to a competitive system
• Worst Acceptable Level
• Worst acceptable, not worst possible
• May relate to use by novices
• Should equal or better current level
• Eventually, if any observed value on any
  attribute is does not meet this level, the system
  is formally unacceptable

23
Planned Target and Best Possible Levels

• Planned Target Level
• The what you would like level
• Set higher than current level (if known)
• Match or exceed competitors product
• Attributes not yet at this level focus
  development effort
• Best Possible Level
• Realistic state-of-the-art upper limit
• Indicates room for future improvement
• Assumes expert user, best design, best available
  technology

24
Advantages of Usability Specifications

• Part of the management and control mechanisms for
  the iterative refinement process
• Define a quantitative end to the (potentially
  endless) iterative refinement process
• Allows clear assessment of usability during
  iterative prototyping cycles
• Identifies data to be collected, avoids gathering
  unusable or irrelevant data
• Objectively identifies interface weaknesses which
  need further design effort

25
Disadvantages of Usability Specifications

• Measures of specific user actions in specific
  situations
• No reliable technique for setting usability
  specifications
• Different tasks and user groups may need
  different usability specifications

26
Lecture Review

• Definition and purpose
• Usability factors
• Measuring instruments
• Usability specification process
• Setting usability levels
• Advantages and disadvantages