Lesson 1 - Unit 9 Overview

"You may delay, but time will not."

- Benjamin Franklin

As reiterated in the previous Unit,  endogenous behavior is something that comes from “inside” the model and is explained by the relationships within the model. That is, if a system evolves endogenously, it means that the model structure itself is inherently causing the model variables to change with time. 

In order to generate endogenous dynamic behavior, an element must have “memory” or “inertia” with regard to the past.  That is, the current value of the element’s output(s) must be a function of inputs to the element at previous timesteps. 

The most commonly used elements that generate such behavior are the “stock elements” (i.e., Pools, Reservoirs, and Integrators). And as we saw in the previous Unit, when used to create feedback loops, these elements can generate very complex dynamic endogenous behavior.  Other GoldSim elements, however, also generate endogenous behavior.  In this Unit we will discuss two important elements that do so, the Material Delay and the Information Delay.

Delays are common in all real-world systems. Delays represent the reality that in many cases, if you do something now, it's impact will not be immediate, but will be delayed.

Material delays represent the delay in the movement or processing of material (e.g., water, items, people) through a system or process. For example, when you mail a letter, there is a delay before it is delivered.  When you start flowing water into a pipeline, there is a delay before it exits the pipeline. When you put a meal into an oven, there is a delay before it is cooked.

Information delays represent the delay in the collection and/or transmission of information. For example, when you take a measurement of some variable that is required to make a decision, perhaps it takes some time (hours, days) before that information is properly collected and transmitted.  As a result, any actions that are taken in response to that information are not actually based on the present state of the system, but a past state of the system.  

In this Unit we will discuss the following:

• Understanding delays;
• Modeling material delays; 
• Handling recirculating (recursive) logic;
• Modeling information delays;
• The interaction between information delays and feedback loops;
• Using an information delay to simulate a forecast; and
• Adding a forecast to a Proportional Controller.

This Unit has a total of 10 Lessons (including this overview and a summary at the end). The Unit also includes 3 Exercises and several Examples.