This is not a critical literature review; it is directed at identifying interesting and potentially relevant behavioural phenomena (BPs), and so might seem unusually selective. The literature to be presented is drawn from five areas. With the exception of Network Planning, which derives from the discipline of Operations Research, the areas or categories are derived from Cognitive Science in general (see Chapter 2 for a discussion of how this forms the major knowledge base for Cognitive Ergonomics) and various sub-disciplines such as Psychology and Artificial Intelligence (AI) in particular. Since there is no obvious mapping to the set of JAs, the simplest and least confusing way of presenting this diverse collection of observations is by the five categories in turn. Immediately below is a brief justification for the inclusion of each of the five areas. Following this, each is addressed in full. The section ends with a summary of BPs which will then be addressed by the derivation of an initial model.
Network Planning. Network planning is one aspect of a branch of science
known as Operations Research. Operations Research originated in the Second World War and was concerned with applying scientific knowledge, not to the development of new technologies, but rather to the improvement of the utilization of existing ones. It has grown to be a large discipline, encompassing many different areas, one of which is concerned with project management, and might be termed ‘Network
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Planning’. This aspect of Operations Research is included for its concepts and terminology, which will be recruited to the initial model of multitasking.
Multitasking. Although it would be possible to ignore any existing research into
multitasking, and develop a model completely independently, this is not, in principle, the approach which will be used here. However, in practice, there is only very little such research and its contribution will be correspondingly small.
Planning Behaviour. Given the criterion that there should be some degree of
familiarity with the individual tasks which are interleaved, it is further assumed that any interleaving will be coordinated in some way, rather than being purely reactive. Such coordination has been termed planning, and has been the subject of much research. Some of the observations emerging from this research are presented.
Attention & Performance. Multitasking is taken to be the realisation of parallel
tasks as a single serial stream of behaviour. Research in psychology which has addressed the limits of the behavioural capacity of the person has typically been put under the heading of ‘Attention and Performance’. This area of the literature is reviewed as a basis for understanding the nature of this seriality.
Interruption. One of the initial JAs specifies that there are likely to be changes to
the set of tasks being interleaved at short notice. Although this will have an effect on the planning and coordination of the interleaving, it may also, in some circumstances, be considered as an interruption. From everyday experience, interruptions may be expected to have a negative effect on performance. This section is included to attempt to qualify this.
The above categories are very similar to those suggested by Miyata and Norman (1986) in their discussion of multitasking. However, the intention here is not to review these categories in the same way, or to the same depth since they were pursuing a different goal. The context of Miyata and Norman’s review is that of a guide for (computer) system designers, and given the constitution of such a
population, amounts to little more than a constrained introduction to psychology. The present review, in contrast, will take much of psychology for granted (e.g. the
psychology of memory), and will not get into research on ‘action’, for example.
3 . 1 . Network Planning
Network Planning is a convenient, although not necessarily universally accepted, generic term for a collection of particular approaches (Smith, 1971). Historically, its basis is in two techniques, PERT and CPM, originating in the late 1950s. PERT stands for Programme Evaluation and Review Technique, and was conceived to manage the development of the Polaris missile. CPM (Critical Path Method)
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originally addressed chemical plant overhaul. The common objective for the two techniques was to optimise the length of time a project would take. Since then, this technique has been shown to be widely applicable, and able to be used to optimise a project with respect to any one objective (e.g. time, cost, etc). Optimising a project with respect to all of its objectives has proven less achievable in practice.
A Network Planning exercise starts with the construction of the network. This is a set of nodes, representing events, linked together by arrows, representing activities. It is then necessary to calculate the earliest and latest times for each event. From these figures, the event slack can be derived as the difference between the earliest and latest times of the event. A critical event is one for which there is no slack in the time at which it can occur. A critical activity would be one which fell between two critical events, and which was equal in duration to the difference in time between these two events. The chain of critical activities through the network, from the start to the finish, is the sequence of longest duration and as such governs the duration of the project. This sequence is termed the critical path. Only if the time taken by activities on this path is shortened can the total project time be shortened. Shortening the duration of activities elsewhere on the network has no such effect.
Such a network, however, is only (at best) half the story, since it only considers the duration and sequential dependencies of activities. A subsequent stage is required to produce a schedule from such a plan. At this point, the resource demands of the activities must be considered with respect to availability. Knowing the time windows (i.e. the difference between the earliest start and the latest finish) of a pair of activities competing for a particular resource allows one to determine whether they can proceed in parallel. If they cannot, then the resource could be said to be overcommitted and require either levelling (taking advantage of under-use elsewhere), or more of that resource. Resource levelling is usually performed on an ad hoc basis.
Most of the jobs which could be described as multitasking under the current definition have many of the attributes of the projects typically analysed by network methods, albeit on a much smaller scale. They consist of tasks, with sequential dependencies and resource requirements. Perhaps a major difference is in the fact that instances of multitasking may not have clearly defined start and end points, but rather are more arbitrary episodes in an ongoing stream. A definite start point and end point is a requirement in network analysis.
There is no intention to perform formal network analyses in the course of this thesis, primarily for this reason, but also because it is not thought that the effort involved would show any appreciable return for the purposes of understanding multitasking
behaviour. Some of the concepts of network analysis will nevertheless be recruited informally to the model of multitasking. Chief among these will be the concept of
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critical activities and the critical path. Without constructing a network it will not be possible to specify fully the critical path. However, it is often possible to judge informally which activities control the overall job time.