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A Context Model Needed for Complex Tasks

Simon Grant

It is interesting to have models of cognition in everyday activities, but also important to have models of task activity, because it can inform the design of tasks, or tools and interfaces for the tasks. Previous models do account for the structuring of long-term memory in terms of frames, scripts, MOPs (mental organization packets), schemata, or similar concepts; but they do not account for other observed characteristics of human complex task performance, and in particular for the way in which humans move between contexts. Evidence from the study of complex tasks is here reviewed and discussed in support of these points. The model presented in outline here uses the term `context' for the conceptual entity that some features in common with scripts, frames or schemata, but goes beyond these previous models in suggesting that the knowledge necessary for context changing is contained in the context itself, along with the knowledge that is applied directly in the task, rather than being controlled by some separate process. This model has the virtue of pointing towards a reason why humans' task skill is contextual, explained in terms of the cognitive demands of performing a task.

1 Introduction

Everyday activities range from the easy, and the straightforward, to the difficult, and the complex. A model designed to cover simple activities may not be adequate to explain and cover more complex ones, whereas a more elaborate model could well cover simpler activities as well as more complex ones. Tasks done using, or with the aid of, computer systems also have a range of complexity. In the study of human-computer interaction (HCI), it is not surprising that the aim of modelling different human tasks has given rise to models of differing capabilities. A model of the cognition underlying human text-editing tasks may inform better HCI design of text editors, but it is less likely greatly to inform the design of interfaces for tasks of substantially greater complexity, such as process control, traffic control, management of large organisations, etc.

Current models of cognition do not deal fully with the cognition underlying the human performance of such complex tasks. Part of the purpose of this paper is to draw attention to this, and to identify some of the missing parts of the models. Extending the range of models is also important because, if we are to develop principles to inform HCI design for complex tasks, we should be able to base the principles on models that successfully cover the phenomena that have already been widely observed and discussed, for example by Bainbridge [2], [3], Rasmussen [8] and Woods [11]. New theoretical developments could help in the making of models that could ultimately be used in HCI design for complex systems.

In the belief that some aspects of everyday life (such as social and family interaction, and household management) are at least as complex as process control tasks, here we choose to look at the problems of process control. This paper first draws attention to phenomena in process control that pose particular problems for current models. Here, we use the example of protocols from steel process operators, gathered by Bainbridge [2]. The difficulties are then spelled out for some examples of current models of cognition. A new idea for modelling is then proposed, based around the idea of an immediate human cognitive task context. This draws on ideas from previous models, but pays specific attention to the transition between contexts.

2 Characteristics of a complex task

Much has been written on the nature of complex tasks from the HCI viewpoint (see references given above). We will here focus on a relatively early example of this kind of study, that of Bainbridge [2]. The reason for this is that Bainbridge reproduces a long protocol, and on the basis of the protocol data constructs a model of some of the cognition behind the task actions. This is probably one of the fullest accounts of the analysis of a verbal protocol from a complex task.

Protocols do not directly reveal the operator's knowledge structures, in the sense of naming the mental structures and contents. This had previously been held by some psychologists, and is called classical introspectionism by Prætorius and Duncan [7]. We must infer mental phenomena from the protocols. To illustrate the nature of this inference, we shall start with a protocol extract.

In Bainbridge's experiment, the subjects were asked to `think aloud' or `talk about everything you think of'. To give a short extract (not selected for any particular purpose) from the protocol in Bainbridge [2]:

I'm going to have to
I'm going to really have to, yes, manipulate on
I'll leave B and C on full tilt
now I'm going to have to manipulate one of these
if I want to cut down
if I'm going to go over it
4 minutes and I've used 6
I've got A oxidising
These kind of protocol phrases do not make a great deal of sense out of context, and one cannot with any degree of certainty infer the characteristics of process control tasks directly from them.

Bainbridge carefully analysed the patterns to be found in the protocols, looking for recurrent routines or sequences of actions able to explain the phrases. She admits that this is impossible to do in a completely formal way, but she was attempting to be parsimonious in the explanatory framework, while at the same time taking account of human limitations in working memory. For our purposes, it is more important that Bainbridge shows that such analysis to be possible, than whether her particular analysis is the best possible. The fact that such an analysis is both extensive and coherent supports the idea that phenomena of the type described occur.

The model that Bainbridge then constructs is able to account for the majority of the protocol phrases. At the highest level, the model consists of interrelated ``routines'' and ``sequences''. These specify the information sought and used by the human, the basis for the decisions taken, and the conditions under which the subject moves to another routine or sequence. These are seen as the largest units of cognitive behaviour in this particular task that occur as a whole.

The most important aspect of Bainbridge's analysis, for our present purposes, is her identification of some characteristics of her routines and sequences. Her findings are consistent with the findings in another experiment using a task of probably similar complexity, which we shall now briefly consider. In this other study [5], subjects were given a dynamic control task which involved simulated naval mine-hunting. The interface in this task had a facility for turning the information displays on and off, and there was a cost (in terms of game score) associated with the visibility of any piece of information. When the subjects had sufficient practice, they were able to perform the task using only a small number of information sources at one time. These information sources were used regularly at particular stages of the task, and could be seen as supporting the rules for action that were employed during that stage.

The common point in the analysis of these different studies was the identification of cognitive structures that are compatible with human limitations, and that can be invoked to explain at once both the information needs of the task in terms of immediate decisions, and the information needs in terms of decisions to change context. Although other models have described the division of human activities into separate modules (under a variety of names), none have convincingly integrated the explanation of both this and movement between contexts, in a description of a complex task, where contextuality is central to the human strategy.

3 Other models and their lack of perfect match

The aim of this section is not primarily to review previous models, but only to point out briefly, for a few models, where it appears that they do not easily account for the kinds of cognition attributable to process control.


SOAR [6] is a well-known model of general intelligence, although it is not primarily designed as a model of human cognition. It is based on clearly-expressed hypotheses about the structure of an architecture capable of general intelligence, and these hypotheses have certain implications for the way in which SOAR would be capable of modelling cognition in a task like process control.

The hypotheses of goal structure, problem space and universal subgoaling map out a problem-solving strategy in which the context can be identified with the current goal, and on this may depend the problem space and the operators that are appropriate. However, this goal or context structure is of limited flexibility. It appears that the goal or context can only be changed in two ways: firstly by success or failure, back to the parent goal; and secondly by generating a subgoal. This discounts the possibility of switching context in a variety of ways, without return to the former goal or context. To connect this with everyday life, surely the experience of being interrupted and losing track of what one was doing must be nearly universal.

In process control tasks, it is important to allow for interruptions. An alarm must take one out of one's present context, and although memory of the context that was left may remain for a while, there is no guarantee of returning to it.

Another hypothesis of SOAR is to do with control knowledge. According to this, ``Any decision can be controlled by indefinite amounts of knowledge, both domain dependent and independent.'' This invites models of process control which violate established human limits on attention, memory and speed of processing.


A useful summary paper by Anderson [1] takes us through the essential features of these theories, where they are presented as theories of learning, rather than theories of how knowledge is put into action. Questions that follows from this are, firstly, can these theories learn the structures that are suggested by empirical studies of complex tasks? and secondly, would they learn this from the experience that we could expect them to work from?

We may accept that it is possible to create a context-like task structure using a production system, simply because in principle anything that can be modelled at all can be done with production systems. A more important practical question is whether it can be done conveniently. There must be room to doubt this in the case of ACT* and PUPS, since there is no explicit provision for such a structure in Anderson's proposed memory systems, and no detailed theory of how knowledge is used in practice, such as could be applied to complex tasks.


Schank's MOP (mental organization packet) theory [9] is constructed primarily to deal with natural story fragments, as was its predecessor, Schank and Abelson's Scripts [10]. According to Schank [9], MOPs serve to sequence scenes, such that if you are in a certain MOP, then you will a fortiori know what scene will come next. But he does not discuss attention, and the way in which a MOP would be chosen in everyday life at a particular time.

General points

Bartlett's theory of schemata [4] is a theory of memory, rather than learning or action. Theories of learning rest on theories of memory, because one cannot have a theory of learning without having a theory of memory (i.e., what is learned and remembered). It is also difficult to imagine a theory of action without a theory of memory. Therefore, theories of memory will constrain both theories of learning and of action. One of the claims made in this paper is that current theories are not well-matched with the apparent phenomena of process control and other complex tasks. What is proposed below amounts to an enhancement of a theory of what is stored in memory, together with a theory of action closely bound to it. It also invites a theory of learning, but this is not yet developed.

4 An outline model of human cognitive contexts

The situation that is described most easily by the model is of a person engaged in a task (particularly a demanding task), and for whom, at any time, there are a small number of appropriate decisions or actions. This person will be attending to information corresponding to those decisions that are relevant to that situation, and perhaps also other information. This is most clear in well-practiced tasks, where the human has learned both how to perform the task, and what information is needed for it. When we look at less practiced tasks, or novice behaviour, this is much less clear. So, for clarity of exposition, we here start by considering a well-learned task.

Well-learned complex tasks are likely to have well-defined stages, and each stage will have its own relevant information and decisions to be made, as outlined above. What we here (naturalistically) call a `context' may then be thought of as a knowledge structure corresponding to a stage of a task, including those cognitive constituents that are peculiar to that stage, such as the information requirements, the appropriate decisions, etc. The context as here described would then be an obvious candidate for something that is stored in long-term memory, and recalled as a whole, as a viable unit of task strategy appropriate to some stage of some task.

If tasks are organised into contexts, the question arises, how are contexts selected or moved between? One obvious apparent possibility is to have a method (or function) mapping situations onto contexts, in which case the human would monitor the appropriate variables in the situation, so that the correct context could be determined. But there is no compelling reason why contexts need to be fully determined by the situation existing at a particular time only: they could have a dependency on history as well. A real (rather than ideal) thermostat is an obvious example: there is always a small interval between the switching-on temperature and the switching-off temperature, and within these limits one has to know the history of the temperature or the state to determine the present state of the thermostat. Furthermore, the way in which people sometimes get disoriented in a complex task suggests that there is no simple function mapping observable variables onto contexts. If there were, it would permit rapid and effective reorientation, which manifestly often fails to happen.

There is a further objection to theories that have an independent function mapping situations to contexts. What variables should be monitored by such a function at any particular time? There are two possible answers. Firstly, all variables that could cause a context shift are monitored concurrently. This is difficult to reconcile with the limitations of human ability, though it is not unreasonable to assume that some few variables are monitored thus. Secondly, the variables determining context shifts at the lowest level are selected according to a higher-level context. This would mean that the same question would reappear at a higher level. What variables determine the higher-level `meta-context', as we might call it? Neither of these answers appears satisfactory.

But the information necessary to guide the human into the appropriate context must be monitored and processed somehow. Sometimes, the structure of a task may permit this information to be channelled through a medium, or modality, different from the information necessary for action or decision-making within the current task context; but this is by no means universal. The channel used for the current context could also warn of a change of context --- particularly, for example, when the value of a certain variable goes outside the normal bounds appropriate to the context. It then makes sense to consider the mechanism for determining the next context to be part of the current context. It is not necessary to prescribe whether this mechanism uses rules, patterns, triggers or whatever, because it does not alter the form of the model. Suffice it to say that the model needs to account for the process of context changing, as well as the information needed to support that process.

A promising approach to modelling context changing would be to have both context-dependent and context-independent context changing mechanisms. We are all familiar with the way in which immediate physical threat, or other strong emotion, can interrupt a task independently of the stage of the task (after allowing perhaps for differing degrees of concentration). But the transitions between contexts within a task seem to be carried out in a context-dependent manner. We could invoke a hybrid model to account for this, in that the context-dependent within-task transitions could be modelled by a sequential, symbolic process, whereas the context-independent transitions could be modelled by parallel processes, working at a sub-symbolic level.

The essential components of a human context are then: the information that is needed within that context, together with whatever processes or procedures are needed to gather or arrive at that information; some means of deciding when to move to another context, based on some of the information used; and rules that connect (possibly) other information used to the decisions that need to be taken in that context. It is not very difficult to imagine how a wide range of human activities (including much from everyday life) could be described in terms of a context model of this kind.

So far, this model has been described as a model of the operator's mental processes, but it also clearly relates to what the operator could have in mind (though the degree of conscious awareness is unspecified, as with other kinds of mental models). According to the model, an operator will be attending to just those sources of information that are relevant to the context. This includes the information necessary for the rules governing task actions in that context, and also the information necessary for the rules governing the changes of context within the overall structure of the task. This attention could, perhaps, be centred around a mental model in the sense of visual imagery, though one cannot claim that this kind of image is necessary. Also in the operator's mind, at some level, will be some awareness of what the other contexts are that connect with the current context, but the mental structures associated with these other contexts need not be fully brought to mind until the particular context is entered. Thus, if a different model image is required by a different context (as would often be the case), that image would come to mind at the point of switching to its appropriate context.

Reasons for a structure of medium-sized contexts

One of the problems of context-like theories is in setting limits for the size of a context. Clearly, we could construct a reasonable model of people's context structure based only on observation and analysis of people's actual performance (as Bainbridge, op. cit.), but that by itself would give only a weak guide, if any, to predicting the context structure that a human will develop in response to a particular complex activity. One of the potential strongpoints of the theory being proposed is that the combination of rules in context and rules for change of context could form the basis for prediction of the optimum size range for contexts for human use. What is done here is only a sketch of how such a demonstration might be done.

To begin with, we need to note what the cognitively demanding aspects of task performance are, which is related to what the limiting features of cognition are. To do this fully would be a major task, so for the time being let us simply equate the cognitive demands to the limitations on working memory for the items that are central to this model, namely: the information that needs to be monitored at one time; the complexity of the rules (or whatever) governing decisions within the context; and the complexity of the rules governing transition to other contexts. The is plenty of scope for the enhancement of the theory by incorporating better models of cognition here.

We may imagine a range of structures representing task cognition, from, at one end, strategies where all the information is always present --- that is, where there is only one context; to, at the other end, strategies where each decision has a context to itself. If there is only one, large context, all information and rules will be present concurrently, so there is much within-context rule complexity, but no rules necessary for the changing of context. If, on the other hand, there is only going to be one decision rule in each context (and therefore little complexity here), there will be relatively many transitions possible between different contexts, and thus much complexity to the rules governing the transition between contexts. What will happen in between?

As the number of contexts increases, so the size of each context will decrease, in terms of the amount of information necessary and the complexity of decision rules internal to that context. We may conjecture that for a given increase in the size of the contexts (decrease in number of contexts) there would be a more than proportionate increase in the complexity of rules. Intuitively, this would be because in a larger context, the decision rules would have to deal with the extra load of distinguishing between different parts of the context. Going the opposite way along the scale, as we increase the number of contexts (the size of each context decreasing), the complexity of the transition rules is going to increase (from zero). Again, it is not unreasonable to conjecture that as the number of contexts increases, the complexity of the transition rules will increase more than proportionately, intuitively because the number of possible transitions increases exponentially with the number of contexts.

Figure 1: A schematic diagram of a possible relationship between context size and cognitive demand

The resulting schematic graph is shown in Figure 1. When the effects of intra-context and inter-context rules and information are added together, it is reasonable to suppose that there will be some moderate size of context where the total human cognitive demand of executing the task will be at a minimum. This corresponds to the structure that we would expect humans to use, on the basis that humans are likely to opt for the easiest task performance strategy. It should be emphasised that this graph is currently conjectural, and more work needs to be done in establishing its correct form.

5 Implications for computer information systems and HCI

Computer technology offers us the possibility of putting into effect insights gained in the study of the human use of information. If these studies of information use in complex tasks were successful, it might prove possible to design information systems to take into account human context structures, and to present just the right relevant information at the right time. Context modelling studies would provide the basis for assessing what information was minimally necessary at any particular stage in a task, and thus interfaces could be designed that provided no more than what was required. Whereas this may not be an important issue in other fields, confusion caused by excess information has been held partly responsible for incidents such as Three Mile Island.

A key feature of this kind of interface might be the keeping track of human task context, providing information specific to each context, and facilitating the transition between contexts. To enable easy context transition, we could envisage either an automatic system, where the display was changed according to what the system reckoned matched the human context transitions, or else, in a graphical interface, buttons could be provided for effecting a change of display according to context, and perhaps the system could flash the appropriate button when calculations revealed that the human would be likely to want to change context.

A context model could thus help in the design of interfaces to complex tasks.


I am grateful to Angela Sasse for discussion of the form of this paper.


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