Posted by Joel Rahn <jrahn@sympatico.ca>
[ Was Feedback and Behavior (SD5408) ]
Integration over time is always accomplished by 'feeding back' the
previous value of a stock in order to calculate its new value. In some
dynamic modelling methods, this feedback is represented explicitly as a
'self-loop' that exits and re-enters the stock so that the diagram
represents the model equations more explicitly.
SD diagramming conventions don't do this because it clutters up the
diagram to no useful purpose: we know that integration (literally)
transforms time series inputs and the transformation is dynamically
independent of the size of the TIME STEP in a properly formulated model.
So that's our dirty little secret: our system diagrams (causal-loop,
stock-and-flow) do not show everything that the equations are doing to
produce our dynamic results. And here is our powerful, big message:
Feedback rules!
Joel Rahn
Posted by Joel Rahn <jrahn@sympatico.ca>
posting date Tue, 09 Aug 2005 10:02:49 -0400
Integration and Feedback
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