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Usability

VIEWS: 13 PAGES: 20

									                      Usability
• Definition:

   “the capability in human functional terms to be used
   easily and effectively by the specified range of users,
   given specified training and user support, to fulfil the
   specified range of tasks within the specified range of
   environment scenarios.”
                                              (Shackel, 1979)
                User Needs
                 Analysis
                              Conceptual &
                             Physical Design
Problem Defn.                               Guidelines
                                            & Standards

                Analysis                                  Prototyping

                                                                          UI Toolkits
                                   Design
                                                                                      Support &
                                                                                    Documentation
                                               Implementation


                                                                        Testing


                                                                                        Maintenance
      Designing for Usability
                User needs analysis
• User characterisation or Stakeholder analysis
   – user characteristics
      •   physical abilities and skills
      •   general abilities
      •   knowledge
      •   computer familiarity
      •   frequency of use
      •   discretionary use
   – user categories
      •   naive
      •   novice
      •   skilled
      •   expert
      •   casual
                       User needs analysis
• Task analysis
     – goal, device, tasks, sub-tasks, actions


    GOAL                                          TASKS
Produce a letter
                                   Edit letter            Print letter



                            Enter text           Amend              ......

                               ......            ......
   DEVICE
Selecting a device          Move cursor
determines the tasks
                                                                   (Preece et al, p412)
                       Hierarchical Task Analysis
       Plan 0 : 1-2 then according            0 prepare & print
       to requirements: 2, 3, 4, 6            letter using Word
       Prior to quitting: 5
       When session over: 7



       1 start Word      2 enter text       3 format text      4 edit text                    7 exit Word

  Plan 2: according to
  requirements: 1, 2, 3                                               Plan 4: according to requirements
   1 enter text                3 add text
                 2 load file                          1 correct text 2 modify text
 from keyboard                  from file

                                                                           Plan 4.2: according to requirements
                                                                   1 delete block
                                                                       of text

                                                      Plan 4.2.1: 1-2
                                                      1 select text 2 press delete

(adapted from Preece et al, p415)
              Cognitive task analysis
   Method for accomplishing goal of deleting a file

PC-DOS
Step 1   Retrieve from LTM that command verb is „ERASE‟
Step 2   Think of directory name and filename and make it the first
         filespec
Step 3   Accomplish goal of entering and executing a command
Step 4   Return with goal accomplished


Macintosh
Step 1   Accomplish goal of dragging file to trash
Step 2   Return with goal accomplished

                                            (adapted from Preece et al, p421)
                       Tasks
•   Goals
•   Dependency and criticality
•   Current user problems
•   Performance criteria
•   Task criteria
•   User discretion
•   Task demands
        Situational/Domain analysis
• Work context
   –   roles and responsibilities
   –   location of people
   –   communication
   –   structures
• Physical environment
   – location of activity
   – physical objects
• Possible occurrences during system use
   –   equipment
   –   availability
   –   interruptions
   –   surroundings
   –   policy
             Conceptual models
• “The problem is to design the system so that, first,
  it follows a consistent, coherent conceptualisation
  - design model - and, second, so that the user can
  develop a mental model of that system - a user
  model - consistent with the design model”
                                          Norman (1986)
Design                        User‟s
Model                         Model

Designer                       User




           Documentation
              System

              System
              Image
                           Norman and Draper (1986
                  Design models


• Designers‟ view of the system
• Based on:
   – user characteristics
   – users‟ tasks
   – environment
• May be inconsistent, ambiguous, obscure
• Ideally user model should map on to design model
   Mental models (User‟s models)


“…the model people have of themselves, others, the
  environment, and the things with which they
  interact. People form mental models through
  experience, training and instruction”
                                        Norman (1988)
                Mental models


• Mental models enable people to generate
  explanations about a system and to predict system
  behaviour
   – structural models
   – functional models
• Errors and confusion result if actual operation
  differs from mental model i.e. mismatch between
  user model and design model
                System Image
• Design model communicated to user via system
  image
• System image presented by:
  – user interface
  – system behaviour
  – documentation
                The User’s Interaction Cycle

                          Set goal

Form intention to act                  Evaluate interpretation
so as to achieve goal                against what was expected


 Map intention into                      Interpret perception
sequence of actions                   according to expectations


Physically execute                      Perceive state of world
 action sequence



                        THE WORLD
                                                       Norman (1986)
                   Gulf of Execution

                     Consistency
                     Compatibility
                     Predictability



          User                                System
         actions                              actions




User‟s                Observability
goals                 Reversibility

                   Gulf of Evaluation

                                        adapted from Norman (1986)
                         Principles
• Consistency
   – similarity of patterns in tasks, info presentation etc
• Compatibility
   – goodness of fit between user‟s expectations & reality
• Predictability
   – interface should suggest which actions are possible
• Observability
   – current state of system made explicit to user
• Reversibility
   – users should be able to backtrack at will
               Bridging the Gulfs
• Gulf of execution
   – interface should suggest, and prompt user about,
     possible actions
• Gulf of evaluation
   – design so that effects of user action are explicit
   – give clear representation of system state
                    Metaphors
• Enhance predictability
• Assist user in forming accurate mental model
• Effective metaphors
   – features of metaphor should not differ too much from
     features of application
   – metaphor should not have too much „conceptual
     baggage‟                Anderson et al (1994)
   Common system metaphors
Application area    Metaphor        World knowledge

Operating envmt    The desktop        Office tasks, file
                                       management

  Spreadsheets     Ledger sheets      Columnar tables


    Hypertext       Note cards              Flexible
                                          organisation

Learning envmts       Travel            Tours, guides,
                                          navigation

   Multimedia         Rooms          Spatial structure of
                                        buildings

                                   adapted from Preece et al p149

								
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