Question

Although causal loop diagrams are wonderfully useful in many situations, they suffer from a number of limitations and can be easily abused. One of the most important limitations of causal diagrams is their inability to capture the stock and flow structure of systems. Stocks and flows, along with feedback, are the two central concepts of dynamic systems theory.

a.         Explain what is meant by stock and flow and support your answer with examples. (300 words)

b.         Explain how does the system dynamics use a particular diagramming notation for stocks and flows and draw the figure of stock and flow structure of task in project phase (250 words)

Answer 1

Answer a: Stocks and flow are two central concept of dynamics systems theory. Stocks can be measured at one particular time and it represents a quanitity which is presented at that point in time and may have gathered in the past. Flow can be measured over an interval period of time. Flow means contribution in stock / inventory or take ways from stock / inventory. Example of Stocks - Amount of milk in the milk tank at any given point of time is measured in litres. Example of Flows - A milk tank is connected to a tap from which milk is constantly flowing out at 100ml per hour. Then tap can be called as a flow unit.

Answer b: The system dynamics use a particular diagramming notation for stocks and flows. These diagramming notion for stocks and flows are as below:

• Stocks - These are represented in rectangular shape and this means that container holding the product
• Inflow - These are represented in a pipe or arrow that pointing addition into the stock.
• Outflows - These are also represented in a pipe arrow which means subtraction from the stock.
• Valves - These are available to control the flows.
• Clouds - These represents the source of stock and sink of stock. Source of stock means flow originating outside the boundary of the model and sink of source means flow leaving the model boundary drain.They both are assumed to have infinite capacity.