Subject: help in pendulum model

Posted by Jay Forrester on 9/30/2008
In Reply To:help in pendulum model Posted by Karim Chichakly on 9/30/2008

Message:

This is all rather curious. Steve Peterson (see his message) apparently did not know there was a global way to turn off the non-negative conditions. Many others do not know about it. Your described procedure is much too hidden. If non-negative restraint is operating, there should be a conspicuous on-screen warning that it is, along with a button on operating screens to turn it off.


Bank balances can go negative and very misleading things happen as the system reaches these arbitrary non-negative limits. If a bank balance cannot go negative, then one can continue to pay out money from an account that should be in default. Your business users should have an on-screen button to switch off the non-negative behavior so they can observe the consequences of their inadequate modeling.


Until I gave up using STELLA for this and several other reasons, I would be trapped by unexpected behavior until I remembered the non-negative characteristics of STELLA.


The non-negative restraints are very bad system dynamics practice. If you feel you must have this deviation from good modeling practice, then it would be better to have the default be a model that is cleanly described by the equations, and perhaps use the above suggested continuously visible warning and a button to toggle between the two conditions.


Jay Forrester

Follow Ups:

help in pendulum model - Karim Chichakly 9/30/2008 
help in pendulum model - Paul Newton 9/30/2008
help in pendulum model - Bill Braun 9/30/2008