System Dynamics Applications / 1

1 COMPLEXITY AND DYNAMIC BEHAVIOUR

1.1 ABOUT COMPLEXITY AND COMPLEX PROBLEMS

The world we live in is complex. In ‘The Fifth Discipline: The Art and Practice of the Learning Organisation’ Peter Senge (1990) explains that complexity is characterised by both detail and dynamics. Detail complexity suggests that there are a myriad of elements, component parts, or actors. There are far too many for us to contemplate at any one time. Dynamic complexity is a consequence of the mechanisms by which the elements, component parts or actors are connected. In ‘Business Dynamics’, John Sterman (2000: 22) explains that dynamic complexity arises because systems are dynamic, tightly coupled, governed by feedback, non-linear, history-dependent, self-organising, adaptive, counter-intuitive, policy-resistant, and characterised by trade-offs.

We might conceptualise the connections or interrelationships of a complex problem existing within a complicated web. As with a spider’s web we cannot tug on one part without transmitting signals in the form of vibrations throughout all parts of the web. The extent to which these vibrations are felt depends upon the structure of the web, the distance from the point of disturbance, the strength of the connections, and the strength of each individual strand of the web. The vibrations may also be amplified when the web is excited under specific conditions. Extraordinary growth in amplitude (resonance) can be produced by a combination of inherent structural characteristics of the web, the energy of externally applied forces, and the frequency of their application.

It is the extent and shape of this metaphorical web, including the strength of interrelationships between connected component parts that defines what we refer to in system dynamics modelling of complex problems as structure. Generally speaking, if we change the structure of a complex system (or problem) we change its behaviour. It is the structure which determines how the system behaves.

The system or human activity system, where people are involved, is said to exist to produce some purposeful outcome. Vibrations and disturbances are continually transmitted across the web structures of complex problems. These take the form of material or physical flows, information, or perceptions of these (when people are part of the system). Though we do not always recognise them, they are there. They occur in every real-world problem.

People in a crowd might jostle each other deliberately or inadvertently. Commercial airliners queuing to land at a busy airport must be controlled so that they do not encroach on each other’s flight paths. In each of these examples, messages are being passed back and forth, and each of the component parts reacts (albeit in different ways) to stimulus, or deliberate actions to deliver management and control.

If we limit the number of people that can congregate in a given place, provide more space or some form of policing, the problem is changed and the resulting dynamics of group behaviour will be quite different. In the air traffic control example we might increase the safety distance between aircraft or impose fare cost penalties to dissuade all but the most determined airlines, or their passengers, from choosing to arrive at our busy airport at peak times. This would serve to change the problem and could effectively reduce competition for airspace at peak times.

In dynamic problems, the structure does not need to comprise complicated or highly non-linear elements to produce complex behaviour. Complex behaviour patterns can be produced by connections of elements or component parts that individually are very simple. How and why this occurs is explained and demonstrated in this chapter.