GOMS%20Analysis%20

1
GOMS Analysis Web Site Usability

• Melody Y. Ivory (UCB CS)
• SIMS 213, UI Design Development
• April 15, 1999

2
GOMS Analysis Outline

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

3
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

4
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
5
MHP Revisited

• Card, Moran Newell (1983)
• principles of operation
• subsystem behavior under certain conditions
• e.g. Fittss Law, Power Law of Practice
• ten total

Adapted from slide by Dan Glaser
6
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
7
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
8
MHP Subsystems

• Motor processor
• input response from WM
• carry out response

Adapted from slide by Dan Glaser
9
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
10
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
11
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
12
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
13
Original GOMS (CMN-GOMS)

• Card, Moran Newell (1983)
• Engineering model of user interaction
• task analysis (how to knowledge)
• Goals - users 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)

14
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

15
Text-Editing Example
16
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)

17
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
• Normans Action Cycle

18
Normans Action Cycle
Goals
Evaluation Evaluation of interpretations Interpre
ting the perception Perceiving the state of
the world
Execution Intention to act Sequence of
actions Execution of sequence of actions
GOMS
The World
19
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

20
Text-Editing Example (KLM)
21
Variants of GOMS

• Natural GOMS Language (NGOMSL)
• more rigorous than CMN-GOMS
• uses cognitive complexity theory (CCT)
• user and system models
• mapping between users 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

22
Text-Editing Example (NGOMSL)
23
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

24
Text-Editing Ex. (CPM-GOMS)
25
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

26
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

27
Web Site Usability Outline

• The Spool Study
• Major Implications

28
The Spool Study

• Jared Spool et al. (96-97)
• Web Site Usability A Designers 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

29
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?

30
Sites

• 9 popular sites
• Cnet - technology resources
• Disney - original redesigned
• Edmunds - car truck resources
• Fidelity - investments
• Hewlett Packard (HP)
• Inc. - original redesigned
• Olympics - 96 Olympics (expired)
• Travelocity - travel resources
• WebSaver - annuity information

31
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?

32
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?

33
Comparison of Sites

• How successful users were at finding information?
• Sites that were expected to do well fared poorly
  and vice versa
• Disney Cnet (graphically intense)
• Edmunds - mostly text

34
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

35
Spool et alsWeb Site Usability Issues

• Classify each issue
• information, navigation, graphic or other design
• very rudimentary
• Total each category

36
Ivorys Web Site Usability Issues (Preliminary)

• Web site usability ? information findability

37
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

38
Web Site Usability Issues

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

39
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

40
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

41
Major Implications

• Web sites arent like software
• software
• success with product implies preference
• Web
• success on site does not imply preference
• content is important