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| Aug 25 2009, 1:49 AM EDT (current) | chachabooth | 27 words added, 11 words deleted |
| Aug 14 2009, 9:26 AM EDT | chachabooth | 496 words added, 1 photo added |
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Stock and flow diagrams are another tool that is used to illustrate the structure of a system. Systems thinkers contend that “All systems, everywhere, consist of these two kinds of concepts – stocks and flows – and none other.”[1]
An example of a system is a bathtub. Here we can see what stocks and flows are and how the system works:
Adapted from Greene Curry, Cathy, Getting Started. Five Lesson Plans to Help Teachers Introduce System Dynamics to their Students, 1995, p. 10. <http://www.clexchange.org/ftp/documents/system-dynamics/SD1996-03GetStarted5Lessons.pdf> (accessed 13 August 2009)
The stock is the water in the bathtub, the flow is the water running from the faucet and running out of the bathtub. We can define (see Greene Curry, p. 9):
You can’t have a stock without a flow and vice versa. Stocks and flows cannot be separated. The units of a flow always represent a rate of change: liters of water running per minute, dollars flowing into and out of your bank account per month. For stocks, the unit is the total accumulation,accumulation that is independent of the rate of change.change, e.g. the total amount of water in your tub.
Another key element in stock and flow diagrams is the one of feedback. In the bathtub example from above, this is not obvious. Imagine that you want to fill the bathtub only until a certain point in order to safe water in your home. This means you need to control the inflow into the bathtub – how does that happen? You observe the bathtub and get a visual feedback about the water level in the tub. When the water approaches the mark, the feedback means that you start closing the faucet.
Numerous examples of simple stocks and flows can be found around us: a bank account, a financial report (with the balance sheet being the stocks and the profit and loss statement being the flows that cause the stocks to change), the degree of frustration in a group is a stock that is changing due to pressures in the environment (flows). (See here Forrester, Some Basic Concepts in Systems Dynamics, p. 7.) This is of course very basic examples, complex systems develop when several stock and flow diagrams come together. the flow into (or out of) your bank account
References:
Forrester, Some Basic Concepts in Systems Dynamics, Sloan School of Management at the Massachusetts Institute of Technology, 29 January 2009, p. 7. <http://www.clexchange.org/ftp/documents/system-dynamics/SD2009-02SomeBasicConcepts.pdf> (accessed 13 August 2009).
Greene Curry, Cathy, Getting Started. Five Lesson Plans to Help Teachers Introduce System Dynamics to their Students, 1995. <http://www.clexchange.org/ftp/documents/system-dynamics/SD1996-03GetStarted5Lessons.pdf> (accessed 13 August 2009)
Notes:
An example of a system is a bathtub. Here we can see what stocks and flows are and how the system works:
Adapted from Greene Curry, Cathy, Getting Started. Five Lesson Plans to Help Teachers Introduce System Dynamics to their Students, 1995, p. 10. <http://www.clexchange.org/ftp/documents/system-dynamics/SD1996-03GetStarted5Lessons.pdf> (accessed 13 August 2009)
The stock is the water in the bathtub, the flow is the water running from the faucet and running out of the bathtub. We can define (see Greene Curry, p. 9):
A stock is an accumulation of something (money on your bank account, tea in a cup, etc.)
A flow is an action or movement – a rate of change. If there is a flow of something, there must be an associated build-up or depletion. The flow changes the amount in the stock.
You can’t have a stock without a flow and vice versa. Stocks and flows cannot be separated. The units of a flow always represent a rate of change: liters of water running per minute, dollars flowing into and out of your bank account per month. For stocks, the unit is the total accumulation,accumulation that is independent of the rate of change.change, e.g. the total amount of water in your tub.
Another key element in stock and flow diagrams is the one of feedback. In the bathtub example from above, this is not obvious. Imagine that you want to fill the bathtub only until a certain point in order to safe water in your home. This means you need to control the inflow into the bathtub – how does that happen? You observe the bathtub and get a visual feedback about the water level in the tub. When the water approaches the mark, the feedback means that you start closing the faucet.
Numerous examples of simple stocks and flows can be found around us: a bank account, a financial report (with the balance sheet being the stocks and the profit and loss statement being the flows that cause the stocks to change), the degree of frustration in a group is a stock that is changing due to pressures in the environment (flows). (See here Forrester, Some Basic Concepts in Systems Dynamics, p. 7.) This is of course very basic examples, complex systems develop when several stock and flow diagrams come together. the flow into (or out of) your bank account
References:
Forrester, Some Basic Concepts in Systems Dynamics, Sloan School of Management at the Massachusetts Institute of Technology, 29 January 2009, p. 7. <http://www.clexchange.org/ftp/documents/system-dynamics/SD2009-02SomeBasicConcepts.pdf> (accessed 13 August 2009).
Greene Curry, Cathy, Getting Started. Five Lesson Plans to Help Teachers Introduce System Dynamics to their Students, 1995. <http://www.clexchange.org/ftp/documents/system-dynamics/SD1996-03GetStarted5Lessons.pdf> (accessed 13 August 2009)
Notes:
[1] Forrester, Jay W., Some Basic Concepts in Systems Dynamics, Sloan School of Management at the Massachusetts Institute of Technology, 29 January 2009. < http://www.clexchange.org/ftp/documents/system-dynamics/SD2009-02SomeBasicConcepts.pdf> (accessed 13 August 2009).
