A Guide to GOMS Model Usability Evaluation using NGOMSL

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A Guide to GOMS Model Usability Evaluation using
NGOMSL
Handbook of Human-Computer InteractionSecond,
completely revised editionM. Helander, T.K.
Landauer, P. Prabhu (eds.)
Presented to Dr. Rossons Usability Engineering
classon 3/24/99by Steve Burnett
Presentation can be found at www.ee.vt.edu/burnet
t/GOMS.ppt
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GOMS Away!
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Presentation Overview

• Why use GOMS?
• What are GOMS?
• How to use GOMS.
• What GOMS cant do.

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Why Use GOMS?

• Provide a cost effective alternative to iterative
  testing
• Engineering model provides for solid numerical
  output and reusability

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What are GOMS?

• Goals, Operators, Methods, Selection Rules
• Representation of how to do it knowledge of the
  user.
• Provides a method for analyzing the complexity of
  a user task

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GOMS Family of Models

• Several different implementations of the GOMS
  concept
• Range from very simple (keystrokes) to very
  complex (cognitive mapping)
• The examples discussed in this presentation use
  Natural GOMS Language (NGOMSL)

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GOMS Family of Models (2)

• NGOMSL uses a natural language format
• NGOMSL is intended to be interpreted by a human,
  not a computer

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Goals

• Something that the user tries to accomplish
• Syntax
• ltverb noungt
• Examples
• delete file
• highlight text

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Operators

• Actions executed by the user
• Syntax
• ltverb noungt
• Same as goals
• Example
• press key

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Operators (2)

• External Operators
• Observable by an experimentor
• Example Pressing a mouse button
• Mental Operators
• Mental actions performed by user
• Example Remembering a command word

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Operators (3)

• High-level Operators
• Can be broken down
• Example Logging into a system
• Primitive Operators
• Simple operations
• Example Press key

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Methods

• A sequence of steps to accomplish a goal
• Syntax
• Method for goal ltgoal descriptiongt Step 1.
  ltoperatorgt Step 2. ltoperatorgt Step n.
  ltoperatorgt Step m. Return with goal accomplished.

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Methods (2)

• Example
• Method for goal move a directory. Step 1.
  Accomplish goal copy a directory Step 2.
  Accomplish goal delete a directory. Step 3.
  Return with goal accomplished.

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Selection Rules

• Route control to the appropriate method to
  accomplish a goal
• Syntax
• Selection rule set for goal ltgeneral goal
  descriptiongt If ltconditiongt Then accomplish
  goal ltspecific goal descriptiongt If ltconditiongt
  Then accomplish goal ltspecific goal
  descriptiongt Return with goal accomplished.

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Selection Rules (2)

• Example
• Selection rule set for goal move the cursor If
  destination visible on screen Then accomplish
  goal moving with arrow keys If destination not
  visible on screen Then accomplish goal moving
  with scroll barsReturn with goal accomplished.

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How to use GOMS

• A task analysis is used to identify user task
  goals
• Goals defined and broken down
• Complexity is analyzed by counting operators that
  were executed

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GOMS Example

• Compare deleting a file on a Macintosh and at a
  DOS prompt
• The goal is the same on both systems
• Delete file

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GOMS model for Macintosh

• Method for goal delete a file. Step 1.
  Accomplish goal drag file to trash. Step 2.
  Return with goal accomplished.
• Method for goal drag item to destination. Step
  1. Locate icon for item on screen. Step 2. Move
  crusor to item icon location. Step 3. Hold mouse
  button down. Step 4. Locate destination icon on
  screen. Step 5. Move cursor to destination
  icon. Step 6. Verify that destination icon is
  reverse-video Step 7. Release mouse
  button. Step 8. Return with goal accomplished.

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GOMS model for DOS

• Method for goal delete file. Step 1. Recall
  that command verb is ERASE Step 2. Think of
  directory name and file name and retain as first
  filespec. Step 3. Accomplish goal enter and
  execute a command. Step 4. Return with goal
  accomplished.
• Method for goal enter and execute a command.
  Entered with strings for a command verb and one
  or two filespecs. Step 1. Type command
  verb. Step 2. Accomplish goal enter first
  filespec. Step 3. Decide If no second filespec,
  goto 5. Step 4. Accomplish goal enter second
  filespec.

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GOMS model for DOS (2)

• Step 5. Verify command Step 6. Type
  ltCRgt Step 7. Return with goal accomplished.
• Method for goal enter a filespec. Step 1. Type
  space. Step 2. Decide If no directory name,
  goto 5. Step 3. Type \ Step 4. Type directory
  name. Step 5. Decide If no file name, return
  with goal accomplished. Step 6. Type file
  name. Step 7. Return with goal accomplished.

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What GOMS cant do

• GOMS cant provide the task analysis
• It is difficult to analyze the steps that are
  being taken in a users mind (internal operators)
• GOMS cant analyze the feel of an interface

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Conclusions

• GOMS is a very powerful tool for analyzing user
  tasks on a system.
• GOMS provides non-objective data during the
  process of interface design and analysis
• GOMS is not a replacement for user testing