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An Introduction to System Dynamics

Partners' Institution
Södertörn University
Duggan, J., 2016. An Introduction to System Dynamics, in: System Dynamics Modeling with R, Lecture Notes in Social Networks. Springer International Publishing, Cham, pp. 1–24.
Thematic Area
Development studies
This chapter presents the important concepts underlying the system dynamics modeling method. Following an initial definition of the term model, a summary of a successful system dynamics intervention is described. The key concepts of system dynamics – stocks and flows – are explained. The process for simulating stock and flow models – integral calculus – is described, with an example of a company’s customer base used to illustrate how stocks change, through their flows, over time. A summary of dimensional analysis for stock and flow equations is provided before the second feature of system dynamics modeling – feedback –is presented. The chapter concludes by summarizing the system dynamics methodology, which is a five-stage iterative process that guides model design, development, test and policy design.
Relevance for Complex Systems Knowledge
The article provides a foundation for doing systems dynamics. It starts by distinguishing between possible outputs to policy makers, such as absolute or conditional precise predictions or conditional imprecise projections.

It provides an example on Population Health policy, specifically the Global Polio Eradication Initiative. Due to shortfall of funding, the policies discussed hwere to continue with the eradication plan or fall back on a control plan. Systems modelling could demonstrate to policy makers what the likely outcome of the choices would be. It also showed that the purpose of modelling is important and that it require interdisciplinary skills.

The article then continues to present the main concepts of system dynamics:

Stocks and flows are the main building blocks and characterize the status of the system. Stock change by the flow in or out of the system. Stock can be presented in numbers of items. Flows are presented as units of stock/ time period. Stock  may be presented as containers and flows as pipes leading in or out of the container. There may be a diserd stae of stocks and flows referred to as the dynamic equilibrium

Integration - is the main mathematical model used insystems dynamics.

Feedbacks is what regulates the level of stocks and flows. Positive feedback loops increases the in flow to the stock or decreases the outflow, while negative decreases the inflow and increase the outflow.

Modelling feedback loops is one of the main tricks. The dynamics modeling process is essential. Starting with articulation of the problem, moving to propose an Dynamic hypthesis, building a simulation model and test it  and then finally design and evaluate policies. There are possibilities to feed ack between each of the steps.
Point of Strength
The article is a straightforward description of system dynamics modelling with examples. Could be used in a pedagogic together with other material that complicates it further. But good as a starting point.