System Dynamics Applications / 2 / 2.2 / 2.2.1

2.2.1 Building Necessary And Sufficient Representations

When confronted by real-world problems we must face choices about the depth of detail we will attempt to represent in our models. Models must be simpler than the real-world problem situations they help us to understand. The real world we might attempt to analyse can never be defined and described in any absolute sense. Consequently, modellers, analysts, or the client group who own the problem, bring to focus their particular perspectives of both the problem and the real world.

The problem situation, once expressed, is actually expressed in terms of the perspectives on the real world that the interested parties bring to the problem analysis. To define the relevant systems (and subsystems) demands taking the specific views of those closest to, or having greatest knowledge of, those systems. By the time we build specific conceptual models, each will incorporate one view of the world—the unquestioned world view, or weltanschauung, as it is described by Checkland.

That one very specific view contains the most important aspects, the key elements that should give us the insights we need. This is critical. We can then simulate our particular representation of real-world behaviour using the simplified model, and so produce sets of necessarily simplified conclusions about the real world we would not be able to come to if we had to contend with all the detail and dynamic interconnectedness of the real world.

In this book, systems thinking and system dynamics modelling building blocks are combined with systems engineering concepts as the bases for building the models we need. For example, through re-use of components parts (modules or building blocks), each designed with specific functionality in mind, and close attention to management of the interfaces between the component parts we can build correctly functioning models. Each of these is developed for a specific purpose with specific representations of particular real-world problems.

The conceptual model(s), at (4) in our chosen modelling methodology (Figure 2-1) are compared with the problem situation (2). At this stage (5) we determine whether or not we have built the right model to represent the real-world problem as we have defined it.

We must also determine whether the model is a sufficient representation for our purposes. That is, we must test the model, ensuring as far as possible that it is correctly constructed and behaves correctly.

We must extensively test each module, sector, and model to gauge their sensitivity to input values and make judgements about their behaviour in comparison with the real-world problem situation as we have expressed it. Once we are convinced that the model is both necessary and sufficient for our purposes, and has been tested as far as practicable, we move on to designing feasible changes to the problem situation and taking action to remedy the problem situation.

At this point it is necessary to highlight that by virtue of their nature, complex dynamic human activity system problem situations are rarely corrected. Changes we might make can be beneficial but are unlikely to be complete or absolute: we should consider them as temporary.