STELLA - Using Models to Cope with Complexity

STELLA HINTS:

Here are a few hints to help you start to design a conceptual model with STELLA. Below, I will use the example from class when I talk about STOCKS, FLOWS, INFLUENCES...

• Every stock should have a flow in and a flow out. You can think of these simply as "Increase" and "Decrease". What influences the increase and the decrease is the real work.
• Many stocks won't flow into or out of other stocks. They will just have an Increase and Decrease flow and will be connected to everything else by influences. A few stocks will flow into other stocks, but you need to think about whether that is appropriate for your case. For example, just because HARES eat PLANTS, that doesn't mean that the PLANT stock flows into the HARE stock. The effect of the PLANTS stock would influence such things as HARE BIRTH RATE or HARE DEATH RATE, but not the HARE stock directly.
• Flows can only be attached to (i.e., flow in or out of) Stocks, not influences.
• You can link an arrow from an Influence to other another influences or to a flow. Not TO a stock.
• You can link an arrow from a stock to an influence. For example, the size of the hare population might have an effect on its death rate (thru disease, overcrowding, etc.), so you could link an arrow from HARES stock to HARE DEATH RATE. Then there would be a link from the HARE DEATH RATE to the HARE DECREASE flow which comes out of the HARES stock.
  NOTE: STELLA doesn't let you use duplicate names. If you decide that you want generic "INCREASE" and "DECREASE" flows for most of your stocks, you can't name them that way. You have to give them slightly different names. For example, you could number them (Incr01, Incr02, etc.) or you could be more descriptive (HareIncr, PlantIncr), etc.