INTERFACES PRINCIPALES DEL SISTEMA WEB

A great deal of planning research has been conducted in the context of AI and has been concerned with the development of computerised planners. Some of this research has been concerned with developing optimal planning systems, whilst other research has been aimed at mimicking human decision making and thus advancing our understanding of it. In the present context, the interest is clearly biased towards this what and why of planning behaviour, and away from the how (i.e. discussion of backward and forward chaining etc).

Hayes-Roth and Hayes-Roth (1979) is a useful example of planning research in AI. They present observational data gathered from a planning exercise plus a cognitive model of planning, using the mechanisms of the Hearsay-II system. The specific details of the study and the model will not be discussed here. There are however, several more general points which emerge from their paper. Rather than propose a hierarchical, successive refinement model of planning, they choose to model planning as an opportunistic process. A plan could thus be conceived of as existing in a 2- dimensional space. One dimension would be time, the other some measure of completeness or degree of detail. Importantly, they observe that planning does not necessarily proceed strictly forward in time, but rather a partially incomplete plan may consist of some complete (i.e. specified in detail) elements interspersed (on the time dimension) with other elements specified in less detail.

Although Hayes-Roth and Hayes-Roth acknowledge that planning alone is only half the story, the other half being the implementation or execution of the plan, they do so in a way common to much AI planning research. This typically sees planning and execution as two separate and successive events, i.e. a plan is something which is created in its entirety beforehand, and then executed. It is worth stating, in this respect, that their own model produces a plan which cannot be realistically executed. Fortunately, there are more recent movements against this divorce of planning and execution and possibly one might speculate that such an unrealistic plan would be avoided in this way (because it would be constantly revised during execution). The theme of the construction of a plan bound up with its execution is taken up by Suchman (1987). Central to her argument is the notion of situated action. For her, all actions are situated - that is to say they exist only in the context of a particular set of concrete circumstances and because these circumstances can never be fully anticipated the actions are therefore essentially ad hoc. Such a belief reduces the role

Chapter 5

of plans in behaviour from complex constructions to simpler orientation devices and similarly reduces the effort an organism might be expected to expend in devising them.

The two above points - that it might be more accurate to think of plans as simple, partial devices, the construction of which is intimately bound up with their own execution - are echoed by Young and Simon (1987). They introduce the concepts of horizontal and vertical incompleteness in plans. A vertically incomplete plan is described as one in which the level of description is still higher than the primitive actions which the executor can carry out and thus requires further refinement. Horizontal incompleteness in a plan refers to the possibility that not all steps of a multistep plan may be specified. In general, they state, "a partially incomplete plan can exhibit both kinds of incompleteness, freely intermingled".

Whilst one of the reasons for incompleteness in plans is thought to be the

unpredictable nature of the world, they suggest that another reason for favouring partial plans is the limited mental workspace capacity of the human (see discussion of Attention and Performance in Section 3.3 below).

Johannsen and Rouse (1983) proposed a new concept in planning in terms of a dichotomy between what they term event driven and time driven planning. Their proposals arise from experimental work using aircraft pilots in a simulator, in which the subjects were exposed to various missions of controlled character. The main measure taken was a verbal self rating of depth of planning on a scale of 1 to 5, every 30 seconds. Subsequent analysis of this data with respect to the nature of the mission at the time was interpreted in terms of the two types of planning. Using the frames model of Minsky (1975), they describe time driven planning as monitoring the execution of a script - so this type of planning might be expected to account for increased depth of planning as an anticipated event was approaching in time. Event driven planning, on the other hand, is described as updating a script, or creating a new one. This type of planning would be expected to occur in response to an unanticipated event.

Finally, in respect of plans, it is worth considering the contribution of Byrne (1981). The assertion of this paper is that plans exist in memory (possibly therefore one might use the term script or frame), but constitute more than just a set of instructions for some complex act, but also serve as a mental representation whereby associated information may be accessed. This conclusion is based on the observation that when, for example, housewives are asked to list the ingredients for lemon meringue pie, they do so in the order that they are used - the inference being that subjects are mentally executing the relevant plan in order to access the desired information. Thus information associated with the performance of a plan may be bound up with the

mental representation of that plan, and may be accessed by the mentally simulated execution of that plan.

To summarize the important points of this admittedly limited sample of the literature on planning behaviour, there are suggestions that a plan is not necessarily a very low level specification (in terms of actions) even in its most complete form, and is likely to be constructed in both time driven and event driven (opportunistic) manners such that it will probably be both horizontally (time dimension) incomplete and vertically (detail) incomplete. Further, it is acknowledged that plans can be remembered from one occasion to another, and therefore need not be constructed afresh but rather modified as necessary. Finally, plans may be mentally executed, and this may serve as a means of recalling information associated with that plan.

.3 . Attention & Performance

The goal of this section is to provide a working approximation of the serial nature of the human which underlies the need for a set of parallel tasks to be realised as an interleaved sequential stream. It is acknowledged, however, that such seriality is not necessarily absolute, and it may be more precise to think in terms of the constraints on performing activities in parallel. The attention and performance literature is vast and so some means of achieving this desired approximation, without an exhaustive review, is necessary. This is provided by Reason (1986), in the form of a ‘consumer guide’ to what he terms Framework Models of Performance.

He contrasts such framework models with local models in psychology in terms of many characteristics. For example, he suggests that whereas local models tend to be predictive and refutable by experiment, framework models tend to be descriptive and more subject to paradigm-shift, and if the primary function of local models is to disambiguate theoretical questions, then that of framework models is more directed towards emphasising the general points of agreement between models and supporting research aimed at limiting the generalisations. The characteristics of such framework models, particularly the broad agreement on certain points, suggest that they should be considered further in the current context.

Reason reviews six of these framework models, which will not be repeated here. Out of this, he concludes that there are several areas of commonality, particularly relevant here are the concepts of a) restricted workspace, and b) controlled and automatic modes of processing.

The concept of a restricted workspace refers to the observation that people are only able to be mentally concerned with one task at a time, whether it be rehearsal for remembering, or visual search of novel stimuli. For example, a great deal of research into the properties of this system has been conducted using an interference paradigm,

in which the subject is encouraged to try to attend to a second task simultaneously with the first. The finding is that performance on both tasks suffers in such

circumstances; the inference being that one task is interfering with the other’s use of a restricted workspace. An everyday example might be trying to listen to two separate conversations at once. This property is typically modelled in terms of a structure, under the name of ‘Working Memory’, however, to avoid association with particular uses of this term, this thesis will recruit the concept under the name of ‘Mental Workspace’.

The other relevant concept, that of controlled versus automatic modes of behaviour, refers to the need for the above Mental Workspace to be involved in controlling behaviour. Much of our behaviour is carried out in a so called automatic mode, without the influence of the Mental Workspace, in which we are not consciously aware of each action performed and each piece of information heeded. Such behaviour is typical of tasks which have been practised and become routine. In contrast to this is a controlled mode, typified by novel or difficult tasks. In this mode, we are consciously aware of our movements and our decisions to take particular courses of action. Controlled mode behaviour is often slower than automatic behaviour, but has the advantage of being very much more flexible - automatic, learned routines are thought to be fairly specific, whereas behaviour in controlled mode is thought to be guided by the circumstances in real time. This coordination of guidance is provided for by the restricted capacity Mental Workspace. From the earlier discussion of the limited capacity of this structure it follows that a person could only realistically be engaged in one task at a time which required such control. The supposed benefit of evolving automatic processing for routine

behaviours is that it frees up the precious Mental Workspace for other things, generally what might be termed parallel mental activity. The limitation of being concerned with just one task at a time then applies to the use of the Mental Workspace in a controlled mode, rather than strictly to the person as a whole. This issue will be taken up later when the initial model is described.