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GOMS Analysis Web Site Usability

VIEWS: 133 PAGES: 41

									    GOMS Analysis &
    Web Site Usability

    Melody Y. Ivory (UCB CS)
SIMS 213, UI Design & Development
           April 15, 1999
    GOMS Analysis Outline

 GOMS at a glance
 Model Human Processor revisited
 Original GOMS (CMN-GOMS)
 Variants of GOMS
 GOMS in practice
 Summary




                                    2
             GOMS at a glance

   Proposed by Card, Moran & Newell in
    1983
    – apply psychology to CS
      » use user model (MHP) to predict
        performance of tasks in UI
            task completion time, short-term memory
             requirements
    – applicable to
      » user interface design and evaluation
      » training and documentation

                                                       3
Model Human Processor (MHP)
         Revisited
   Card, Moran & Newell
    (1983)
    – most influential model of
      user interaction
    – 3 interacting subsystems
       » cognitive, perceptual & motor
       » each with processor & memory
              described by parameters
                 – e.g. capacity, cycle time
       » serial & parallel processing




                           Adapted from slide by Dan Glaser   4
                  MHP Revisited

   Card, Moran & Newell
    (1983)
    – principles of operation
       » subsystem behavior under
         certain conditions
             e.g. Fitts‟s Law, Power Law of
              Practice
       » ten total




                          Adapted from slide by Dan Glaser   5
             MHP Subsystems

   Perceptual processor
    – sensory input (audio & visual)
    – code info symbolically
    – output into audio & visual
      image storage (WM buffers)




                     Adapted from slide by Dan Glaser   6
             MHP Subsystems

   Cognitive processor
    – input from sensory buffers
    – access LTM to determine
      response
       » previously stored info
    – output response into WM




                       Adapted from slide by Dan Glaser   7
            MHP Subsystems

   Motor processor
    – input response from WM
    – carry out response




                   Adapted from slide by Dan Glaser   8
    MHP Subsystem Interactions

   Input/output
   Processing
    – serial action
       » pressing key in response to
         light
    – parallel perception
       » driving, reading signs &
         hearing




                        Adapted from slide by Dan Glaser   9
               MHP Parameters

   Based on empirical data
    – word processing in the „70s
   Processors have
    – cycle time ()
   Memories have
    – storage capacity ()
    – decay time of an item ()
    – info code type ()
       » physical, acoustic, visual &
         semantic


                        Adapted from slide by Dan Glaser   10
        Perceptual Subsystem
             Parameters
   Processor
    – cycle time () = 100 msec
   Visual Image Store
    – storage capacity () = 17
      letters
    – decay time of an item () =
      200 msec
    – info code type () = physical
       » physical properties of visual
         stimulus
             e.g. intensity, color,
              curvature, length

                           Adapted from slide by Dan Glaser   11
     One Principle of Operation

   Power Law of Practice
    – task time on the nth trial
      follows a power law
       » Tn = T1 n-a, where a = .4
       » i.e., you get faster the more
         times you do it!
       » applies to skilled behavior
         (perceptual & motor)
       » does not apply to knowledge
         acquisition or quality




                        Adapted from slide by Dan Glaser   12
    Original GOMS (CMN-GOMS)

 Card, Moran & Newell (1983)
 Engineering model of user interaction
    – task analysis (“how to” knowledge)
      » Goals - user‟s intentions (tasks)
            e.g. delete a file, edit text, assist a customer
      » Operators - actions to complete task
            cognitive, perceptual & motor (MHP)
            low-level (e.g. move the mouse to menu)




                                                                13
                  CMN-GOMS

   Engineering model of user interaction
    – task analysis (“how to” knowledge)
      » Methods - sequences of actions (operators)
            based on error-free expert
            may be multiple methods for accomplishing same
             goal
               – e.g. shortcut key or menu selection
      » Selections - rules for choosing appropriate
        method
            method predicted based on context
    – explicit task structure
      » hierarchy of goals & sub-goals
                                                              14
Text-Editing Example




                       15
         CMN-GOMS Analysis

   Analysis of explicit task structure
    – add parameters for operators
       » approximations (MHP) or empirical data
       » single value or parameterized estimate
    – predict user performance
       » execution time (count statements in task
         structure)
       » short-term memory requirements (stacking
         depth of task structure)
    – apply before user testing (reduce costs)
                                                    16
     Limitations of CMN-GOMS

   No directions for task analysis
    – granularity (start & stop)
   Serial v.s. parallel perceptual
    processing
    – contrary to MHP
 Only one active goal
 Error-free expert performance
    – no problem solving or evaluation
       » Norman‟s Action Cycle
                                         17
           Norman’s Action Cycle

                               Goals


         Execution                            Evaluation
        Intention to act               Evaluation of interpretations

      Sequence of actions              Interpreting the perception

Execution of sequence of actions   Perceiving the state of the world
GOMS



                               The World
                                                                       18
              Variants of GOMS

   Keystroke-Level Model (KLM)
    – simpler than CMN-GOMS
       » six keystroke-level primitive operators
             K - press a key or button
             P - point with a mouse
             H - home hands
             D - draw a line segment
             M - mentally prepare to do an action
             R - system response time
       » no selections
       » five heuristic rules (mental operators)
    – still one goal activation                      19
Text-Editing Example (KLM)




                             20
              Variants of GOMS

   Natural GOMS Language (NGOMSL)
    – more rigorous than CMN-GOMS
       » uses cognitive complexity theory (CCT)
             user and system models
                – mapping between user‟s goals & system model
             user style rules (novice support)
       » task-analysis methodology
       » learning time predictions
       » flatten CMN-GOMS goal hierarchy
             high-level notation (proceduralized actions) v.s.
              low-level operators
    – still one goal activation                                   21
Text-Editing Example
     (NGOMSL)




                       22
           Variants of GOMS

   Cognitive-Perceptual-Motor GOMS
    (CPM-GOMS)
    – activation of several goals
      » uses schedule chart (PERT chart) to
        represent operators & dependencies
      » critical path method for predictions
    – no selections




                                               23
Text-Editing Ex. (CPM-GOMS)




                          24
           GOMS in Practice

 Mouse-driven text editor (KLM)
 CAD system (KLM)
 Television control system (NGOMSL)
 Minimalist documentation (NGOMSL)
 Telephone assistance operator
  workstation (CMP-GOMS)
    – saved about $2 million a year


                                       25
                        Summary

   GOMS in general
    – The analysis of knowledge of how to do a task in terms of the
      components of goals, operators, methods & selection rules.
      (John & Kieras 94)

    – CMN-GOMS, KLM, NGOMSL, CPM-
      GOMS
   Analysis entails
        » task-analysis
        » parameterization of operators
        » predictions
               execution time, learning time (NGOMSL), short-
                term memory requirements
                                                                      26
    Web Site Usability Outline

 The Spool Study
 Major Implications




                                 27
             The Spool Study

   Jared Spool et al. (96-97)
    – Web Site Usability: A Designer‟s Guide
   Usability on the Web
    – shift from most E-Commerce studies
       » converting clicks into sales
    – focus on people finding information to
      make decisions (purchases)
       » sites that provide info to support sales


                                                    28
Usability Testing of Web Sites

   Sites (Interfaces)
    – 9 popular sites (products & info)
   Tasks
    – information retrieval (4 types of tasks)
   Users
    – familiar with Web browsers
   Not a formal usability study
    – experiment design, number of users &
      experience, testing procedure?
                                                 29
                      Sites
   9 popular sites
    –   C|net - technology resources
    –   Disney - original & redesigned
    –   Edmund‟s - car & truck resources
    –   Fidelity - investments
    –   Hewlett Packard (HP)
    –   Inc. - original & redesigned
    –   Olympics - 96 Olympics (expired)
    –   Travelocity - travel resources
    –   WebSaver - annuity information
                                           30
                         Tasks

   “Scavenger Hunt” Tasks
    – retrieve information to answer questions
    – simple facts
      » locating information
            e.g. Can you get a Honda Accord for under
             $15,000?
    – comparison of facts
      » locating two pieces of information plus a
        comparison
            e.g. Which has better acceleration, the Jeep
             Cherokee or Toyota Land Cruiser?
                                                            31
                         Tasks

   “Scavenger Hunt” Tasks
    – judgment
      » locating information plus a decision
            e.g. Do you think a used Ford F-10 is safe enough?
    – comparison of judgment
      » locating multiple pieces of information plus a
        decision
            e.g. Which convertible is the best deal for under
             $20,000?




                                                                 32
         Comparison of Sites

   How successful users were at finding
    information?
    – Sites that were expected to do well
      fared poorly and vice versa
      » Disney & C|net (graphically intense)
      » Edmund‟s - mostly text




                                               33
         Overall Site Usability

   Room for improvement
    – finding information is an intensely
      frustrating experience for users
      » enormous time and effort to answer simple
        questions (simple facts) even on small sites
      » users give up without finding information




                                                       34
           Spool et al’s
      Web Site Usability Issues
   Classify each issue
    – information, navigation, graphic or other
      design
    – very rudimentary
   Total each category




                                              35
      Ivory’s Web Site Usability
         Issues (Preliminary)
   Web site usability    information
    “findability”




                                         36
      Web Site Usability Issues

   Navigation design
    – number of links, location of links
    – within-page, wrapped, embedded, image
      links
   Graphic design
    – too much white space, unrelated or
      distracting graphics



                                              37
      Web Site Usability Issues

   Information design
    – no support for comparisons, poor
      readability
   Other design
    – waiting for server




                                         38
          Major Implications

   Graphic design neither helps nor
    hurts
    – users may report as issue, but does not
      correlate with users‟ success
   Text links are vital
    – downloading delays




                                                39
           Major Implications

   Navigation and content are
    inseparable
    – shell strategy leads to many generic
      links
   Information retrieval is different
    than surfing
    – implies different design approach
       » surfing - need to attract users
       » information retrieval - help users find
         information, more focused
                                                   40
             Major Implications

   Web sites aren‟t like software
    – software
      » success with product implies preference
    – Web
      » success on site does not imply preference
            content is important




                                                    41

								
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