Two Loops, Three Loops, or Four Loops: Pedagogic Issues in Explaining Epidemic Dynamics

摘要

How many feedback loops, and of what type, control the behavior of an epidemic? A search of the literature discovered two-, three-, and four-loop versions of the basic epidemic model in introductory system dynamics materials. How can the same behavior be explained with such different feedback structures? Can they all be right? This paper analyzes the three basic model structures and discusses implications for system dynamics pedagogy.We conclude that either the two- or four-loop versions of the basic epidemic model are acceptable representations, with the two-loop version recommended for beginners; the three-loop version of the system is never correct. In addition, we suggest that the development of incorrect representations such as the three-loop epidemic model can be avoided if standard system dynamics modeling practice is followed formulating a dynamic hypothesis to explain the observed behavior.