1. 1st Meeting Steps of Modeling and Problem Definition
6th System Dynamics Summer School, July Andrew Ford, Instructor, Intermediate Track
1st Meeting Steps of Modeling and Problem Definition
Introductions:
Students,
Assistants,
Instructor
Schedule and Book Website

2. Steps of Modeling Step 1 A is for get Acquainted
Get acquainted with the client, the system and the problem
Step 2
B is for Be specific
Be specific about the dynamic problem
Step 3
C is for Construct
Construct the sock and flow diagram
Step 4
D is for Draw
Draw the causal loop diagram
Step 5
E is for Estimate
Estimate the parameter values
Step 6
R is for Run
Run the model to get the reference mode
Step 7
S is for Sensitivity
Conduct sensitivity analysis
Step 8
T is for Test
Test the impact of policies

3. Another View

4. Parameter Estimation
To decide how much effort to put into estimating a given parameter value, one ought to know how sensitive the behavior of the model is to the value of that parameter. Yet to know that, one must run the model, and that requires parameter values. To resolve the dilemma, the modeler picks some values rather quickly and simulates the model. Initial estimates are made carefully, to be sure, with as much concern for accuracy as can be easily mustered, but keeping in mind that one can always go back and estimate more carefully if it makes a difference.

5. Sources of Information
the measurement of real values may be difficult or even impossible. Under these circumstances the value used in the model, legitimately, may be a straight-out guess. Of course, such a value cannot be regarded and treated in the same way as other parameters in the model. It must be treated tentatively and its role can be evaluated using techniques of sensitivity analysis. (Kitching, Systems Ecology)
Include your guess if it is better than zero

6. Good modeling is an iterative process of trial and error
Why don’t we just get it right the first time?
Iteration reveals mistakes
Iteration brings out the good ideas in the group
Iteration increases group understanding
Good Advice from Ed Roberts: Complete first iteration within 25% of time

7. A good pattern of Iterative Modeling
This is Fig from MTE and from the PhD Thesis by Allyson Beall-King

8. Alternative style of modeling (NOT recommended)

9. Concluding Suggestions for the modeling process:
Iterate early and often
Two, Four, Six, Eight, Don’t forget to Iterate

10. First Two Steps in Modeling
A is for get Acquainted with the client, the system and the problem B is for Be specific about the Problem

11. Problem Definition
Identify the client or clients group Ask the clients: What is the Problem? (the dynamic problem!!) Work with client group to describe the “Reference Mode” or “Target Pattern”
Does a “Target Pattern” seem troubling to you?

12. S-Shaped Growth vs. The Overshoot: Often the goal of sustainability policies

13. Example of S-Shaped Growth
Annual Visitors to Glacier National Park

14. More Examples of S-Shaped Growth
Yellowstone Nat. Park
Yosemite Nat. Park

15. Tips to arrive at a useful Reference Mode
Learn from separate drawings by all members of the Client Group Which pattern should be selected to start model building ? Keep other patterns in mind for later in the project (they may guide the development of a support* model with an entirely different time scale and target pattern.) *For examples, see “Simulating fast and slow dynamics” from the 60th Anniversary Issue of the System Dynamics Review, July 2018