Title: HumanComputer Interaction
1Human-Computer Interaction
2Lecture Overview
- Definition and purpose
- Usability factors
- Measuring instruments
- Usability specification process
- Setting usability levels
- Advantages and disadvantages
3USABILITY
- Previously defined in terms of
- Learnability
- Throughput
- Flexibility
- Attitude
4ISO 9241 Usability Definition (1990)
- The effectiveness,
- efficiency and
- satisfaction with which
- specified users can achieve
- specified goals in a
- particular environment.
5Functional and Usability Specifications
- Functional specifications are central to ensuring
system functionality - Usability specifications are central to ensuring
system usability
6Specification 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
7Purpose of Setting Usability Specifications
- Establish when an interface is good enough
- When to stop iteration
8Common Usability Factors
- Speed of operation
- Completion rate
- Error free rate
- Satisfaction rating
- Learnability
- Retainability
- Advanced feature usage
Main software usability measures identified by
IBM
9Speed of Operation
- A measure of the
- delay between
- initiating an action and
- achieving the users goal.
10Completion 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.
11Error-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?
12Satisfaction Rating
- A subjective judgement by the user.
- Can be quasi-quantified using a rating scale.
- Very important for user acceptance.
13Learnability
- Difficult to quantify
- May take long time to judge
14Retainability
- By definition, requires time to test
- Could use benchmark tasks, performed at intervals
(weeks, months, etc.)
15Advanced 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)
16Time Dimension for Usability Factors
- Initial performance
- Long-term performance
- First impression
- Long-term user satisfaction
17Measuring 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
18Usability 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
19Sample 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)
20Example 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
21Setting 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
22Current 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
23Planned 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
24Advantages 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
25Disadvantages 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
26Lecture Review
- Definition and purpose
- Usability factors
- Measuring instruments
- Usability specification process
- Setting usability levels
- Advantages and disadvantages