Taking a Causal Loop Diagram to a Stock-Flow Model

[jbh]

I have gained some experience over the past year with Causal Loop Diagrams (CLD) and am now trying to take a CLD to a Stock/Flow diagram. Can you provide me with some tips or point me to a good resource that will help me distinguish which variables in my model should becomd stocks, flows, and variables?

Thanks,
JBH

[LAUJJL]

hi

To choose between auxiliary, flow and stock it is necessary and sufficient to understand what is a stock, a flow and an auxiliary. If you do not you will not be able to construct any model.
To understand the difference, just study the Vensim user guide and the modeling guide.
A good introduction is to go to the SD society web site
www.systemdynamics.org then distance learning and the Road map and the introduction to systemdynamics is largely
enough to understand the differences between the three variables.
Regards.
J.J. Laublé

[jbh]

Thanks for the reply. I am working through the guide now. Can you ever have an arrow coming out of a flow that goes to an auxiliary variable and not a stock?

Thanks,
jbh

[LAUJJL]

You can get an arrow going from a rate or flow to any auxiliary variable except to the stock it is currently feeding and to any other stock. Arrows can come out from any variable (auxiliary, stock or flow) to other auxiliaries and flows except to stocks. The only variables that feed stocks are flows and the initial value of stocks.
Regards.
J.J. Laublé

[bob@vensim.com]

On connecting arrows out of flows - this is something that people often argue about. Usually, a flow connects only to the levels it is changing. Rarely, a flow might be connected to another flow or to an auxiliary variable (see for example a coflow).

As a general rule you should probably resist connecting an arrow off a flow. That is, try at least once to see if there is an alternative way to formulate things. If not, so be it. Vensim will actually let you do anything you want - this is just good modeling practice.

Bob Eberlein