Feedback Loop Diagrams Different Diagrams for Different Stages

Feedback Loop Diagrams Different Diagrams for Different Stages of the Modeling Process Step 1 A is for get Acquainted Get acquainted with the client, the system & the problem Step 2 B is for Be specific about the dynamic problem Step 3 C is for Construct the sock and flow diagram Step 4 Draw the causal loop diagram D is for Draw Step 5 E is for Estimate the parameter values Step 6 R is for Run the model to get the reference mode Step 7 S is for Sensitivity Conduct sensitivity analysis Step 8 T is for Test the impact of policies These slides focus on the conventions for “word and arrow” diagrams in Step 4; their the purpose is to check if the model has the necessary feedback loops to generate the “Reference Mode. ” A 2 nd type of causal loop diagram can be useful in the initial “Brain Storming. ” These diagrams can explode in size, perhaps growing to include 50 or more loops. They are not discussed here. My experiences with clients have led me to leave the word and arrow diagrams at home when talking with clients. The slides conclude with an alternate, much more effective, way to show clients how the key loops are simulated in the model.

Population Model Example

Home Heating Example

Why Draw These Diagrams? • To see the feedback loops • There are two kinds of loops – Positive feedback loop – Negative feedback loop • The diagrams are for thinking and discussion (not for simulation)

Positive Feedback Loops • • • Can often lead to exponential growth Or they can lead to rapid decline If the changes are good, then “virtuous circles” If changes are bad, “vicious circles” The loop acts to magnify the effect of outside disturbances • 0, 2, 4, 6 (any even number) of – arrows in the loop

Negative Feedback Loops • Can often lead to exponential decline • Or they may serve as control loops – with an explicit goal (i. e. , engineered like a thermostat) – or with an implicit goal (a natural system) • The loop acts to negate the effect of outside disturbances • 1, 3, 5, 7 (any odd number) of – arrows in the loop • Tend to be much more prevalent than positive feedback loops

Figs 9. 4, 5, 6. Guidelines for labeling an arrow as + or -

Supply & Demand: Example of coupled loops. Two Negative Feedback Loops: Do these act to “negate” the effect of outside disturbances?

Boom Town Housing Problem (my first problem after graduate school)

Look for the Feedback and you will see the “Big Picture” And you will probably by the ONLY person to simulate the key loop!

D is for Draw the Causal Loop Diagram The previous examples start from scratch. The diagrams to illustrate the way we label each arrows and the way we label each loop. Now, suppose we already have a model and we proceed to step 4: D is for Draw the feedback loops in the model.

Population Model Diagram Start with the stocks, then add the flows

Then explain the flows, best to start with the easy flows

Next: explain the births

When Do We Draw the Causal Loop Diagrams?

S-Shaped Growth Examples • Flowers Model • Sales Company Model • Epidemic Model

Loops in the Flowers Model

Loops in the Sales Model

Loops in an Epidemic Model

Plug in Symbols for “interpretation” Positive Feedback Loops (+) Negative Feedback Loops (-)

Advice for Beginners • Avoid “action words. ” The “action is in the arrows, not in the words” • Avoid “too many arrows” – your goal is to show the loops, not the clutter.

Exercises: Find the Errors

How Many Loops?

Label the Loops in this Diagram

OK, Now you know how to draw causal loop diagrams for sharing within the System Dynamics Community But how should we show the key feedback loops in the model to the client? especially a client that advises us to Please leave those word and arrow diagrams at home. Show the Stock & Flow Diagram instead. and Name the Loops!

Naming the 3 loops in the Sales Model

Naming the 3 loops in an Epidemic Model

Naming the 2 loops in the Model of Office Space Construction

Final Example: The Striga Model (thanks to Timothy Silberg, Michigan State University) Naming the Loops is one of several things we can do for clarity