Lecture E2 (2021-03-09): Making Simulations More Realistic, Part 2 - Delays, Fixed and Smoothing

In this lecture, we describe how to implement delays in stock-and-flow diagrams in Vensim and Insight Maker. We focus on two specific types of delays -- fixed delays (as in transport delays) and smoothing/averaging delays (as in exponential decay). The lecture is motivated by a hypothetical exercise of modeling the ingestion process -- where food enters the mouth, travels down the esophagus (fixed delay), and then gets processed in the stomach (smoothing delay) as nutrients move into the bloodstream. Much of this lecture is devoted to building up intuition about what a "delay" parameter is in a smoothing delay -- this parameter captures the inertia (or sluggishness/resistance to change) of systems. That allows us to re-introduce the "time constant" again.