Subject: Stocks and Flows

Posted by Steve Wantuch on 3/4/2004
In Reply To:Stocks and Flows Posted by Lees Stuntz on 3/2/2004

Message:

Lees,
I would like to add further examples of Stocks and Flows to your Debt/Deficit example. Some of those might be:

GDP as a stock and producing activities as flows,
AIDS infected victims as a stock and the people becoming infected as a flow,
Global warming as a flow affecting global temperature as a stock,
Discrimination in society as a stock added to by discriminating activities

The list is virtually endless and and the language metaphor of Nouns and Verbs serves to help students define the applicable system.

In a larger sense, the idea of stocks and flows relates very well conceptually to a language metaphor. Stocks are the nouns which can be described as being accumulated or depleted in a model. Flows are the verbs which are usually described with an "ing" ending . Another good test for whether something is a stock or a flow in a model can be made by looking at the item when all action ceases. If there is an accumulation which remains, then this would imply a stock. If there is no accumulation and the item falls to zero it is indicative of a flow.

One other point is that in order for a student to affect change, the leverage points in a model relate to flows. In the above AIDS example, it is only a flow (curing, vaccinating, dying) which is able to to affect the stock of infected victims.

Sincerely,

Steve Wantuch
STELLA Customer Service/Tech Support Representative
High Performance Systems, Inc.
46 Centerra Parkway, Suite 200
Lebanon, NH 03766-1487
Orders: www.hps-inc.com
Phone: 603-643-9636 - Fax: 603-643-9502

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