Subject: Connections between models and reality

Posted by Richard Turnock on 3/6/2004
In Reply To:Connections between models and reality Posted by Niall Palfreyman on 3/5/2004

Message:

What a great simple description of the process:"...move back and forth between reality and model."

Students and Teachers new to dynamic modeling have difficulty with the mechanics of the software so you have to take time to dialogue about math, navigation, left and fight mouse clicks, vocabulary, etc. After you get past the basic introduction, assuming the student is now familiar with the software, what is the process to "move back and forth between reality and model?" The process students follow determines their behavior.

Process steps:
(Wayne Wakeland's class at PSU teaches the process outlined below using book by Sterman, Business Dynamics)
1. Determine Reference Behavior Pattern (RBP). For example, an increase in global temperature caused by anthropogenic carbon dioxide emissions. Graph the data over time from measurements. For some modeling projects that might be based on qualitative variables such as social capital or self-esteem.

2. Hypothesis: State the Expected Model Behavior (EMB). For example, the EMB is shifting feedback loop dominance resulting in S-shaped behavior of global temperature over time due to the delays in the system. (Hopefully you're starting off with the bathtub model instead of this example.)

Then you get into boundaries, assumptions, problem formulation, testing assumptions, starting with a working model, and comparing working model to reference behavior. (these steps and those below may not be in right procedural sequence)
Making sensitivity runs of the model for a range of values to identify limitations or problems with the model compared to the RBP.
Debugging a model might happen at any step in the process of making changes to the model.
Calibrating the model variables.
Verification and validation of the model are two key process steps.
Each proposed improvement or change to the model needs to go through each of these steps above.

The process students follow determines their behavior. If students use a flawed process to "move back and forth between reality and model," then they will not understand the difference between a stock and a flow.

Richard

Follow Ups:

Connections between models and reality - Niall Palfreyman 3/8/2004 
Connections between models and reality - Richard Turnock 3/9/2004
Connections between models and reality - Niall Palfreyman 3/11/2004
Connections between models and reality - John Sterman 3/8/2004