SD modeling process

One drawback of using a computer to simulate systems is that the computer will always do exactly what you tell it to do. (Garbage in - Garbage out)To master systems dynamics, a one must acquire many skills:

have a general idea of how the system should behave to test the accuracy of the computer simulationgenerate a valid model of the system to be analyzedenter the model correctly into the computermake the model clear so that others can understand its structure

How to build the model?

Rewarding and challengingNo set way to build a modelEach model is different

How to build the model?

Rough guidelines:1. Define the purpose (goal / what you want do with the model)2. Define the system you wish to model (boundaries / not everything)3. Identify key variables in the system (what affects your system the

most / carefully name your variables)4. Describe the behavior of the key variables (how they behave in

real life / not exact answer but some) 5. Identify the stocks and flows in your systems.6. Map them in VENSIM7. Define the flows (each flow must be defined only by the stocks and

constants)8. Include quantitative information (enter equations and data)9. Run the model (test the behavior)10.Evaluate the model (look at each relationship)11. Improve the model

SD modeling process

Common Mistakes:the solution DT too largea flow that should be bi-directional is one-way (stock can be + or -)a stock is incorrectly defined as non-negativeincorrect flow directionincorrect equations

SD modeling process

While modeling, you may find yourself consistently making the same mistakes, or misunderstanding certain concepts.

Do not be discouraged (take notes of your mistakes)Very common mistakes:

(a) Dimensional inconsistency (conversion coefficients)

(b) Creating stocks that can not accumulate

Conceptualization

1st stage of the model building process is the hardest and called conceptualization. It involves:

(1) definition of model purpose(2) definition of model boundaries and key variables(3) description of the behavior(4) graphing of basic mechanisms ( feedback loop )

Modeling WorksheetProblem Area: _________________________________________________________________Purpose of Model: ______________________________________________________________Model Audience:________________________________________________________________Model Boundary initial component list:________________________________________________(Mark each stock with S and each flow with F)Reference Modes:

Assumptions on reference models: __________________________________________________Hypothesis for the dynamics of the model behavior:_____________________________________Any interaction which is essential to the behavior mode must be included inside the system

boundary.

Use of Generic Structures

We learned a number of generic structures:

1. First order positive feedback2. First order negative feedback3. S-shaped growth4. Oscillating systems (higher order

feedback)

Fitting problems to generic structures may lead to incorrect models.

Use of Generic Structures

Keep in mind that STOCKS IN A MODEL MUST REPRESENT ACCUMULATIONS within the system, and FLOWS REPRESENT CHANGES IN THE STOCKS.Remember that many structures can produce similar behaviors.Generic structures should not be used blindly without thinking about the structure of the specific system being modeled.

Sensitivity analysis

Choice of specific values of some parameters in a model is very important in building a valid system dynamics model.Sensitivity analysis is a powerful tool in model building and evaluation. It is used to determine how ‘sensitive’ a model is

to changes in the value of the parameter of the model and to changes in the structure of the model.