2.5 A model of systems modelling
Much, if not most quantitative modelling is carried out in the context of engineering, business and financial studies. These uses of quantitative models are usually not part of a systems approach. Furthermore much of the modelling carried out in systems studies is not quantitative, since issues can often be resolved by using diagrammatic or conceptual models. It is therefore important to clarify the systems context in which modelling in general, and quantitative modelling in particular, will be carried out.
To explore the overall process of using models in systems work, we can use a conceptual model. To start, we need a description of the system that produces models within systems studies, and then we can build a conceptual model of this. A possible definition of the system is:
A system to represent aspects of some situation of interest to a range of stakeholders, to assist those stakeholders to achieve some purpose(s) relevant to them.
This system definition includes three verbs, represent, assist and achieve. These are the ‘front-line’ activities, and implied by these activity verbs, there is a wide set of further activities. Thus, to represent some situation in an appropriate way, it is necessary to:
A know the stakeholders and the purposes
B identify the boundaries around the situation
C choose features of that situation which are important to the stakeholders
D identify possible linkages or relationships between these features
E choose a suitable way to describe those relationships
F test whether the chosen features and relationships are agreed by the stakeholders to be appropriate
G modify B–E if the test in F suggests that inappropriate choices have been made and test again.
The end result of a systems study is often to take action. For systems modelling to assist the stakeholders towards this, then it may also have to:
H indicate the likely outcome of different possible actions, in terms relevant to the stated purpose(s)
I evaluate (i.e. put some value on) the nature of trade-offs between different purposes, so as to indicate the preferred option.
The set of verbs listed above represents the necessary activities of a systems model, and the logical links between them are given in Figure 7.
Drawing diagrams is often a very important way of carrying out activities B to E in the sequence above.
Activity H possibly needs some further explanation. The outcome of any action is usually some change in certain features of the situation. This can take many forms, but usually results in a change in the behaviour of some item or person. Behaviour, in this sense, has a precise meaning referring to the way something changes over time. It could imply something as obvious as the temperature of a room, displayed as a graph, or it could be something much less quantifiable, such as the level of conflict, or morale in an organisation. The phrase ‘the behaviour of the system’ thus refers to changes over time in those aspects of the system which have been identified as being important or relevant. Where aspects of the behaviour are measurable, each of these aspects is usually referred to as a variable, often qualified as a state variable. The whole set of state variables associated with a system then summarises the state of that system under a given set of conditions, in terms relevant to the agreed purpose of the modelling.
The list of activity verbs and the logical linkage between them does not tell us how to do modelling in practice. In order to do any systems modelling, we need to put some flesh on the conceptual model given, to provide one realisation of the activity of systems modelling.
One such realisation is provided in David Lane, Camilla Monefeldt and Jonathan Rosenhead's paper 'Emergency – but no Accident: a system dynamics study of an accident and emergency department'. To access the article click here to go to the website of The Operational Research Society. Once there enter the search terms 'emergency accident lane monefeldt' in the 'site search' box located in the top bar of the page. The paper should be among the top hits provided.
Click on the 'View document' link below to read David Lane, Camilla Monefeldt and Jonathan Rosenhead, on 'Emergency but no accident'.
To familiarise yourself with the context briefly read the sections up to and including 'Handling complexity with system dynamics' in the 'Emergency – but no accident' paper.
As part of a systems study of a hospital accident and emergency department, the following root definition was produced for one system that the stakeholders thought was relevant to the whole situation:
‘A system to categorise patients entering the accident and emergency department so that hospital resources are committed effectively according to patient needs and patients are treated and sent on appropriately.’
Using the discussion of conceptual models in the preceding text as a guide
identify the important verbs in this root definition
draw up a conceptual model of a system which conforms to this root definition.
There are four actual verbs in the definition: categorise, commit, treat and send on.
These verbs all imply a range of other activities, which need to be specified to produce a useful conceptual model. So, to categorise patients, it is necessary to know the features of the categories, and of the patients. To commit resources, it is necessary to know what resources are available, and to match them to the needs. To treat and send on patients, it is necessary to select treatments, to know what resources they need and to use these resources to carry out the treatments. One version of a more developed conceptual model including these verbs is given in Figure 8.