Usability Principles

1
Usability Principles

• CptS 443
• Human Computer Interaction

2
Usability Principles

• How do we describe/measure usability?
• Learnability
• Time to achieve proficiency
• Flexibility
• Different ways of doing the same thing
• Robustness
• Level of support

3
Learnability

• How easy is the interface to use?
• Predictability
• Synthesizability
• Familiarity
• Generalizability
• Consistency

4
Predictability

• What happens when I press this button?
• Consequence of current action
• Deterministic behavior, no surprises
• Operation visibility
• What operations are available to the user?
• User does not have to memorize permitted actions

5
Synthesizability

• How did I get here?
• Consequences of previous actions
• Immediate honesty
• Results of actions immediately visible
• e.g. WIMP file system
• Eventual honesty
• Results of actions must be inquired
• e.g. command line

6
Familiarity

• Hey, I think I can figure this out!
• How well the system relates to previous
  interfaces/experiences
• Guessability
• e.g. typewriter/wordprocessor
• Affordances
• Intrinsic actions of visual objects
• e.g. buttons are pushed

7
Generalization

• Does this work here?
• Actions learned in one environment also work in
  other environments
• Analogical mapping
• e.g. cut paste
• word processor
• file system

8
Consistency

• Just when I get comfortable with a system, it
  changes!
• Changes in interfaces are disruptive
• Need a good reason to change
• Otherwise, be consistent

9
Flexibility

• How broad is the interface
• Dialog Initiative
• Multi-threading
• Task Migratability
• Substitutivity
• Customizability

10
Dialog Initiative

• Who started it?
• Who interrupts whom
• System pre-emptive
• Core interrupts user
• e.g. Are you sure you want to do that?
• User pre-emptive
• User interrupts core
• e.g. buttons, Esc, Ctrl-C, Ctrl-Alt-Del

11
Multi-threading

• How do I do two things at the same time?
• Support for multiple tasks
• Concurrent
• simultaneous communication
• Interleaved
• one conversation at a time
• Multi-modality
• multiple tasks, multiple channels

12
Task Migratability

• You do that and Ill do this
• Division of labor between the core and the user
• e.g. spell checking
• Who is in control?

13
Substitutivity

• What format should I use?
• Input and output should be available in multiple,
  redundant forms
• Representation multiplicity
• State of the system shown in several different
  but equivalent forms
• Equal opportunity
• No difference between output and input
• e.g. forms, spreadsheets

14
Customizability

• How can I make this interface work better?
• Core or user changes interface dynamically
• Adaptibility
• User changes the interface
• Adaptivity
• System changes the interface

15
Robustness

• How well supported is the interface?
• Oberservability
• Recoverability
• Responsiveness
• Task Conformance

16
Observability

• What is going on?
• Ability to understand the current state
• Browsability - explore the state
• Static/dynamic defaults - memorization
• Reachability - getting from state to state
• Persistence - Quick, look, there it is!
• Operation visibility

17
Recoverability

• Oops!
• Ability to reach goal after a mistake
• Forward recovery
• mistakes are not revokable
• Backward recovery
• Undo
• Commensurate effort
• Difficult to correct, then difficult to do in the
  first place

18
Responsiveness

• What is the system doing now?
• Communication latency
• Stability
• Variations in similar interactions
• e.g. all menus should take the same time to pull
  down

19
Task Conformance

• This doesnt do what I need it to do
• How well the interface supports the users task
  needs
• Task completeness
• How comprehensive the interface covers the users
  tasks
• Task adequacy
• How well the interface supports the task