Participants were shown how one particular model, loosely based on the Limited Enthusiasm Model, was built. This also introduced them to the concepts of stocks, flows, converters and connectors. The full presentation can be downloaded, with the four elements in System Dynamics. Essentially there are
- Stocks. Also called accumulations. These measure quantity. What is accumulated stays there unless there are processes that change it. For example the number of people who belong to a church at any given time could be a stock. It is a static view of the system - what is there now, rather than what happens over time.
- Flows. Also called rates. These measure change. How many people are converted per year could be a flow. It is a dynamic view of the system, that is, how things change over a period of time.
- Connector. These control change, though linking one element to another. They capture the sense of cause and effect. If the people who belong to church cause (that is they are involved in the process of) conversion then there is a connector from Church to conversion. The more in the church, the more conversions
- Converter. These convert one type of quantity into another, so that elements of different types can be linked together. Thus they can appear in causal chains. There are special converters that interface with the boundary of the model and have values that are set from outside. These are the parameters.
It is important to understand that any system has a static view (the stocks - what is there now), a dynamic view (the flows - how things are changing) and a causal view (the connectors - what causes what). People possess these three views of a system but often they are in conflict with each other without them realising it. One purpose of a system dynamics model, and its simulation, is to highlight these conflicts, and guide people to a resolution and a better understanding of the system.
The first model described in some detail was the discipleship model. With the basic stocks of the system constructed from discussions over three meetings the group assigned values to the stocks and to the parameters that control the flows, based on realistic guesses. Thus a static and dynamic view of the system was identified. The causal view was left on hold. A worksheet was constructed with the initial guesses as the parameters. The first guess gave a wide discrepancy between static and dynamic views - note the big difference between the first guess and the second which is where the stocks are calibrated to the flows (attached sheet). Participants were either overestimating the number of spiritually mature people in the church, or underestimating the process to spiritual maturity, or both.
A blank parameter worksheet is available.