Hierarchical Approach in System Dynamics Modeling

Abstract

Traditionally system dynamists used two kinds of modelling approach; top-down approach and bottom-up approach. With top-down modelling approach (feedback loop thinking) modellers first draw a causal loop diagram and then details it down to the stock and rate variables and then auxiliary. Top-down approach forces a modeller to see forest rather than trees, while bottom-up approach forces a modeller to construct forest by planting trees incrementally. In general, system dynamics use both approach in modelling processes simultaneously or sequentially. However, most of previous softwares for SD modelling have focused on the bottom-up approach. The ‘operational thinking’ concept of Barry Richmond is most typical in this trend. In this paper, “a hierarchical modelling” is introduced as a new modelling approach which can integrate top-down approach and bottom-up approach. A hierarchical modelling approach extends the sector view concept proposed recently by Barry Richmond. In the hierarchical modelling approach, a sector can contain many sub-sectors which also can have their own sub-sector. A modeller can use a bottom-up approach or operational thinking in a sector. When a modeller wants to connect variable across sector or to extend a variable into a sector, he can use a top-down approach. In this paper, a hierarchical modelling approach is implemented with an object-oriented programming method. This paper explains why an object-oriented programming is necessary in implementing the hierarchical modelling approach and discusses some problems which should be resolved for developing a more easy and transparent hierarchical modelling approach.