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The GOMS Model
Examples of GOMS

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GOMS at a Glance
Motivation
How To Use GOMS
Types of GOMS
Advantages of GOMS
Disadvantages of GOMS
Examples of GOMS
Applicability to HCI
Project Ernestine
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Some Examples of the GOMS Models

An example of The Keystroke Level Model (KLM)

Consider the text editing task of searching a Microsoft Word document for all occurrences of a four-letter word, and replacing it with another four-letter word. Here we use the notation of Card et. al (1983, p264-265). K represents pressing a key or a button. P represents pointing with the mouse to a target on the display. H represents moving hands to the home position on the keyboard or mouse. M is a heuristic to incorporate mentally preparing for a task. The time intervals are taken from the same source, for an average typist (55 wpm). In the table below, operations are sometimes concantenated and repeated. For example, M4K means "Mental preparation, then 4 Key presses."
Description Operation Time (sec)
Reach for mouse H[mouse] 0.40
Move pointer to "Replace" button P[menu item] 1.10
Click on "Replace" command K[mouse] 0.20
Home on keyboard H[keyboard] 0.40
Specify word to be replaced M4K[word] 2.15
Reach for mouse H[mouse] 0.40
Point to correct field P[field] 1.10
Click on field K[mouse] 0.20
Home on keyboard H[keyboard] 0.40
Type new word M4K[word] 2.15
Reach for mouse H[mouse] 0.40
Move pointer on Replace-all P[replace-all] 1.10
Click on field K[mouse] 0.20
Total 10.2

According to this KLM model, it takes 10.2 seconds to accomplish this task.


An example of Card, Moran, Newell (CMN)

Card, Moran, and Newell GOMS (CMN-GOMS) is the version proposed originally. It adds a formal notation for expressing the goal hierarchy that resembles program form. The hierarchy can decompose tasks into many levels of subtasks, with operators being executed in strictly sequential form. Using the previous examples, this form is as follows:

GOAL:  ICONIZE-WINDOW
    [select GOAL:  USE-MINIMIZE-MENU-METHOD
		MOVE-MOUSE-TO-HEADER-BAR
		RIGHT-CLICK
		MOVE-MOUSE-TO-MINIMIZE-OPTION
		LEFT-CLICK
	    GOAL:  USE-HEADER-BAR-BUTTON
		MOVE-MOUSE-TO-HEADER-BAR-BUTTON
		LEFT-CLICK ]

Selection rules:
User George:
RULE 1:  Use USE-MINIMIZE-MENU-METHOD unless another rule applies
RULE 2:  Use USE-HEADER-BAR-BUTTON if the mouse has one button

GOAL:  EDIT-DOCUMENT
  GOAL:  OPEN-DOCUMENT
  GOAL:  SAVE-DOCUMENT
    [select GOAL: USE-SAVE-MENU-METHOD
		MOVE-MOUSE-TO-FILE-MENU
		LEFT-CLICK
		MOVE-MOUSE-TO-SAVE-OPTION
		LEFT-CLICK
	    GOAL: USE-SAVE-SHORTCUT
		HIT-CONTROL-S-KEYS  ]

Selection rules:
User George:
RULE 1:  Use USE-SAVE-SHORTCUT-METHOD unless another rule applies
RULE 2:  Use USE-SAVE-MENU-METHOD if on machine X

CMN-GOMS also applies a physical model that maps to the K and P operators of KLM. The main difference is that CMN-GOMS places the mental time in verify procedures at the end of a sequence of operators, whereas in KLM this mental time is generally in the beginning. This difference is generally not important.


An example of a partial NGOMSL model, taken from John & Kieras.
Method for goal: Cut text
  Step 1.  Accomplish goal: Highlight text.
  Step 2.  Return that the command is CUT, and 
             accomplish goal: Issue a command.
  Step 3.  Return with goal accomplished.
...
Selection rule set for goal: Highlight text
  If text-is word, then accomplish goal: Highlight word.
  If text-is arbitrary, then accomplish goal: Highlight arbitrary text.
  Return with goal accomplished.
...
Method for goal: Highlight arbitrary text
  Step 1.  Determine position of beginning of text (1.20 sec)
  Step 2.  Move cursor to beginning of text	   (1.10 sec)
  Step 3.  Click mouse button.			   (0.20 sec)
  Step 4.  Move cursor to end of text.		   (1.10 sec)
  Step 5.  Shift-click mouse button.		   (0.48 sec)
  Step 6.  Verify that correct text is highlighted (1.20 sec)
  Step 7.  Return with goal accomplished.
This NGOMSL model predicts that it will take 5.28 seconds to highlight arbitrary text.


An example of Cognitive-Perceptual-Motor GOMS (CPM-GOMS)

CPM-GOMS builds on previous GOMS models by assumed that perceptual, cognitive and motor operators can be performed in parallel. It employs a schedule chart (also known as a PERT chart) to represent operators and dependencies between operators. CPM-GOMS is also known as Critical-Path-Method GOMS. The figure below is an example of a schedule chart for implementing the goal READ-SCREEN, when an eye movement is required. This figure is taken from John & Kieras (1996b.). The sequence which produces the longest path through the chart is called the critical path, and it represents an estimate of the total time required to perform the task. In this case (assuming a simple binary visual signal on the screen), the critical path is 50+50+30+100+50=280 msec. This particular model assumes that visual perception, cognitive operations, and eye movements can occur in parallel.