Card, Moran, and Newell GOMS Model

GOMS model, as proposed by Card, Moran, and Newell (1983), is a description of the knowledge that a user must have in order to carry out tasks on a device or system. It is a representation of the "how to do it" knowledge that is required by a system in order to get the intended tasks accomplished. It describes the Goals, Operators, Methods, and Selection rules needed to perform a task.



GOMS model, as proposed by Card, Moran, and Newell (1983), is a description of the knowledge that a user must have in order to carry out tasks on a device or system. It is a representation of the "how to do it" knowledge that is required by a system in order to get the intended tasks accomplished. It describes the Goals, Operators, Methods, and Selection rules needed to perform a task.


When is a GOMS analysis done and what for?

1. In the design phase.
2. It is a way to characterize a set of design decisions from the user’s point of view.
3. In the evaluation/usability testing phase.
4. It can be used to obtain predictions of learning and performance, and to establish tasks to monitor
5. In the documentation phase.
6. It is a way to describe what the user must learn, and so could be used as a basis for training, and for developing reference documentation.

Goals 

- A goal is something that the user tries to accomplish.
- It is typically described in an action-object pair in the form: 
<verb noun> 

- Goal can be defined at any level of abstraction.
Examples are: 
delete word 
move-left cursor 
insert database-entry 
compose text 
delete file

Operators

- Operators are the actual, basic actions the user executes.
- Like goals, they are also represented as an action-object pair.
- the (intuitive) difference between goals and operators is that the goal is something the user wishes to achieve, while the operator is just something the user executes:
Goal                        Operator
move-left cursor     press-key ←
- Operators can affect the system or only the user’s mental or physical state.


Methods

- Amethod is a sequence of steps that accomplishes a goal.
- A step typically consists of one or more operators (external or mental) or a call to sub-methods to accomplish subgoals.
- The form of a method is as follows:

Method to accomplish <goal description>:


Step 1. <operator>

Step 2. Accomplish <subgoal description>

Step 3. Report goal-accomplished


For Example:
Method to accomplish move file:

Step 1. select-icon file

Step 2. drag-icon destination

Step 3. report goal-accomplished


Selection Rules

- In some occasions, there can be more than one method to accomplish a goal.
- A selection rule helps to choose the appropriate method in the particular circumstance.
- A set of mutually exclusive conditions is described that specify what method should be used in what context.
- Selection rules are defined in sets: a set is associated with a general goal and consists of If-Then rules.

The form for a selection rule is:
Selection rule for <general-goal description>:
If <condition 1> then use <method 1>
If <condition 2> then use <method 2>


- Each <condition n> consists of one or more operators that test

-The working memory.

- The content of the task description.

- Or some external perceptual situation.
- The order of the If-Then statements is not significant.
- But only one of the conditions can be true at a time.

A Procedure for Constructing a GOMS Model 

1. Use a top-down, breadth-first expansion of methods: 

2. Go from the most general user goal to more specific subgoals .

3. All the goals at each level are dealt with before moving to the next level .

4. Use high-level operators and define methods which use these high level operators .

5. Transform each high-level operator into a goal, and repeat the process .

6. If you go breadth-first rather than depth-first it is more likely that you will notice how methods are similar to each other (this will improve consistency).
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