Qualitative Modelling in System Dynamics or What are the Wise Limits of Quantification?

The tradition in system dynamics is that a problem can only be analysed, and policy guidance given, through the aegis of a fully quantified model. In the last 15 years, however, a number of purely qualitative models have been described, and have been criticised, in the literature. The paper briefly reviews that debate and then discusses some of the problems involved in quantification. Those problems are exemplified by an analysis of a particular model which turns out to bear little relation to the real problem it purported to analyse. Some qualitative models are then reviewed to show that they can, indeed, lead to policy insights and five roles for qualitative models are identified. Finally, a research agenda is proposed to determine the wise balance between qualitative and quantitative models. Professor Geoff Coyle has been active in system dynamics for more than 30 years. He has published two textbooks and many papers and consulted widely. He was the first recipient of the System Dynamics Society's Lifetime Achievement Award. Geoff is currently Visiting Professor of Strategic Modelling at the University of the South Bank and an expert advisor to HVR Consulting Services Ltd as well as consulting and teaching on a freelance basis.

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