HUMAN-COMPUTER INTERACTION SECOND EDITION
Dix, Finlay, Abowd and Beale

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Chapter 1 The human 1.6.3 Techniques for evaluation Page 49

1.6.3 Techniques for evaluation

Chapter 3 The interaction 3.2 Models of interaction Page 104

We begin by considering the most influential model of interaction, Norman's execution--evaluation cycle; then we look at another model which extends the ideas of Norman's cycle. Both of these models describe the interaction in terms of the goals and actions of the user. We will therefore briefly discuss the terminology used and the assumptions inherent in the models, before describing the models themselves.

Chapter 3 The interaction 3.2.2 The execution--evaluation cycle Page 105

3.2.2 The execution--evaluation cycle

Chapter 3 The interaction 3.2.2 The execution--evaluation cycle Page 105

The interactive cycle can be divided into two major phases: execution and evaluation. These can then be subdivided into further stages, seven in all. The stages in Norman's model of interaction are as follows:

Chapter 3 The interaction 3.2.2 The execution--evaluation cycle Page 106

Norman uses this model of interaction to demonstrate why some interfaces cause problems to their users. He describes these in terms of the gulfs of execution and the gulfs of evaluation. As we noted earlier, the user and the system do not use the same terms to describe the domain and goals -- remember that we called the language of the system the core language and the language of the user the task language. The gulf of execution is the difference between the user's formulation of the actions to reach the goal and the actions allowed by the system. If the actions allowed by the system correspond to those intended by the user, the interaction will be effective. The interface should therefore aim to reduce this gulf.

Chapter 3 The interaction 3.2.2 The execution--evaluation cycle Page 106

The gulf of evaluation is the distance between the physical presentation of the system state and the expectation of the user. If the user can readily evaluate the presentation in terms of his goal, the gulf of evaluation is small. The more effort that is required on the part of the user to interpret the presentation, the less effective the interaction.

Chapter 3 The interaction 3.2.3 The interaction framework Page 107

As the interface sits between the User and the System, there are four steps in the interactive cycle, each corresponding to a translation from one component to another, as shown by the labelled arcs in Figure 3.2. The User begins the interactive cycle with the formulation of a goal and a task to achieve that goal. The only way the user can manipulate the machine is through the Input, and so the task must be articulated within the input language. The input language is translated into the core language as operations to be performed by the System. The System then transforms itself as described by the operation translated from the Input; the execution phase of the cycle is complete and the evaluation phase now begins. The System is in a new state, which must now be communicated to the User. The current values of system attributes are rendered as concepts or features of the Output. It is then up to the User to observe the Output and assess the results of the interaction relative to the original goal, ending the evaluation phase and, hence, the interactive cycle. There are four main translations involved in the interaction: articulation, performance, presentation and observation.

Chapter 3 The interaction 3.2.3 The interaction framework Page 108

Once a state transition has occurred within the System, the execution phase of the interaction is complete and the evaluation phase begins. The new state of the System must be communicated to the User, and this begins by translating the System responses to the transition into stimuli for the Output component. This presentation translation must preserve the relevant system attributes from the domain in the limited expressiveness of the output devices. The ability to capture the domain concepts of the System within the Output is a question of expressiveness for this translation.

Chapter 3 The interaction 3.10 Summary Page 138

In this chapter, we have looked at the interaction between human and computer, and, in particular, how we can ensure that the interaction is effective to allow the user to get the required job done. We have seen how we can use Norman's execution--evaluation model, and the interaction framework that extends it, to analyze the interaction in terms of how easy or difficult it is for the user to express what he wants and determine whether it has been done.

Chapter 3 The interaction Recommended reading Page 139

A classic text, which discusses psychological issues in designing everyday objects and addresses why such objects are often so difficult to use. Discusses the execution--evaluation cycle. Very readable and entertaining. See also his two more recent books Turn Signals are the Facial Expressions of Automobiles [179] and Things That Make Us Smart [180].

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