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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.


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).

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.

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