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Taking the three main structures individually:
The Problem Representation is thought of as containing the threads - i.e. the set of affordances to be satisfied. In addition, there must also be some representation of the available resources. The threads contained in the Problem Representation are the anticipated sequence of affordances, which may be different from that which eventually happens (the ability only to specify a thread post hoc has been discussed already). Note that this relationship is different from the question of whether there is a break in the temporal sequence of transformations on an object. Such a break reflects the resources allocated to the affordances, and so is a product of the Solution. In this way, the model maintains a separation between what has to be done (in the Problem Representation) and when and how it will be done (in the Solution). The Solution structure must then specify what resources are to be allocated to what affordance, and when. This can be looked at from two points of view. Firstly as the utilisation of a given resource over time, and secondly as the scheduled performance of the sequence of transformations associated with an object. It is reasonable to expect that such a duality might be observable in the way subjects express their intentions - which are taken to reflect the contents of the Solution. (Actually, subjects ought to be able to express the intended utilisation of any of the resources in the system. This is not entirely unreasonable, on reflection. For example, one might say “I will do X then Y then Z”, which would be a statement of the intended use of the self as a resource, or equally, one might say “I’ll use this saucepan for X then for Y”.)
Finally, the Solver. This structure remains unchanged. It is still drawn as a grey box to reflect both the original intention that it should be a ‘black box’ (which was only of interest in terms of its properties), and the later qualification of aspects of its internal workings.
6 . 3 . Mechanism
The much simplified structure of the model has a similarly simple mechanism. This is the basic problem-solution scheme, in which the problem is imperfectly specified, and the solution thus the best estimate at a given moment. A change in the
specification of the problem is taken to result in an automatic validation of the solution. This validation notion is extended to cover the relationship between the problem as it exists in the real world, and its internal representation. This is
represented in the diagram (Fig. 5.2.) by the lower horizontal arrow. The strength of such a scheme is that many separate phenomena blur together, and can be understood in terms of the same mechanism. (That such an apparent strength may in fact be a weakness is discussed in brief in a section to follow.)
The state of the internal Problem Representation with respect to the problem in the world can be understood as the basis for many of the observed phenomena. For example, interruption, monitoring and sharing the set of tasks with other people. It is even observed that the operators explicitly validate their internal model of the world from time to time, especially when about to decide what to do - it would seem a good idea to update one’s representation of the problem before basing any decisions on it. In the light of the specific changes to the conception of tasks and resources,
interruption warrants a special discussion. Previously, the view of tasks was person centred, and so, consequently, was the view of interruption. The decentralisation of the model results in a corresponding decentralisation of interruption. In the new model, interruption has to be modelled in terms of each of the resources in the
system, and thus it is the use of a particular resource which gets interrupted. This can be thought of in general as the use of the particular resource not being as anticipated. Previous observations of interruption are then interruptions of the Mental Workspace and/or Performance System resources. With respect to other resources, it is
interesting to note the case reported above (Section 4.3.4), where the operator interrupts a printer.
General mechanistic issues aside, it is necessary to consider how the model addresses the novel behavioural phenomena observed in the present study. The observation of two stages in interruption has already been discussed in the results section, but at first, such an idea appears contrary to the foregoing discussion of decentralisation, although a prime example of the proposed validation scheme. However, when considered in detail, it does not appear too unreasonable. In such two stage interruption, it is considered that there is a potential separation of the
acknowledgement of the need to divert a resource, and its subsequent actual diversion to another task. Consider an example of interrupting a printer in order to restart the computer. It is perfectly plausible to consider the first stage to be examination of the current state of the printer, and the second stage to be the termination of the current document. Separation would occur in the case where it was deemed worth waiting for the current document to finish.
It would be easy to misunderstand the intended nature of the Mental Workspace resource in the context of such an interruption for two reasons. Firstly, the person must usually become aware of the requirement to interrupt the use of a resource, and secondly, the person is also likely to have to implement the change of use. In answer to the first point, the model makes an assumption about the Mental Workspace
resource, such that at a low level it is not necessarily exclusively focussed, but rather is aware of other things. In respect of the second point, in the model, the act of interrupting itself is a task, and can thus require resources.
The observation of simplicity or ease in deciding planning what to do is modelled simply in terms of a bias or heuristic in the Solver. The action of this structure is bypassed in the case of a learned routine. Observations of behaviour were made in this respect in this study, particularly at the start of the session. Such routine would be produced in the model by the initial specification of the Problem Representation from memory, and a corresponding filling of the Solution from memory. Such a solution would be valid, but the specification of the problem would necessarily be an assumption of normality about the world. Given the complexity of e.g. the
operator’s world and the nature of the backup process, it is not unreasonable that the greatest influence of routine should be at the start when there is the greatest likelihood of normality.
Doing things in passing is modelled simply in terms of the validation schemes. It is assumed that changing the specification of the Problem Representation is the result of both active (e.g. monitoring) and passive processes. A passive updating of the Problem Representation is merely being aware of the surroundings.
Sharing a set of tasks is a particular, special case of needing to maintain an accurate and up to date representation of the problem. Active validation in this respect, as well as the consequences of not doing so have been observed.
The ‘output’ of the model, represented as the upper, right pointing, horizontal arrow, is thus allocations of resources to affordances, which is more than the actual
behaviour of the individual, even though the individual is likely to be involved in the actual physical allocation itself, as has been discussed.
6 . 4 . Summary
This study has contributed to both the horizontal and vertical development of the model. In Section 2, two additions to the scope of the model were proposed, concerning the experience of the operators with the job, and the presence of other operators. The horizontal development of the model in this respect has proved successful, helping to identify such BPs as validation of the Problem Representation. In addition, this cycle in the development of the model has seen an important change in the underlying conception of tasks. This is reflected, along with other data based influences, in the simplified structure of the model. Many of the observed
behavioural phenomena are modelled in terms of the same representation - validation scheme. Unfortunately, there is no way of knowing at this point whether such simplification is a good or bad thing. It may be that its ability to account for many apparently different phenomena results from it being unspecific. This will be partly put to the test in Chapter 7, when an attempt will be made to recruit the knowledge in the model for design. An important consequence o f the revised conception of tasks
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and the decentralisation of the model is that the definition of multitasking can now be applied to a greater range of jobs than before. The next and final study will take advantage of this.
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