REPLY Definition of root cause (SD6812)

[SDMAIL Jack Homer]

Posted by  "Jack Homer" <jhomer at comcast.net>

Jack Harich writes:
> Page 86 of Sterman says "Formulate a dynamic hypothesis that explains 
> the dynamics as endogenous consequences of the feedback structure." 
> This implies that the definition of "dynamic hypothesis" is "the 
> feedback structure" of the model. This seems to mean the entire model.

John Sterman is talking about the essential feedback structure that 
explains the dynamics of the real system, not the dynamics of a computer 
model.  This feedback structure will go into the model, but models 
typically include a lot of other variables, as well, either to spell out 
the details that are glossed over in the dynamic hypothesis, or to add 
some other variables that the audience for the model would expect to see 
in there, even if it turns out later that those additional variables are 
not so important after all. (Remember, the dynamic hypothesis may turn 
out to be incorrect and have to be revised.)

Based on his postings, I suspect that Jack Harich may be confusing 
looking for root causes in the real system with looking for root causes 
in a computer model.  The latter is pretty easily done in most models, 
with all of the tools of model analysis we have at our disposal.  The 
former, however, is not so easily done, and comes down to the process of 
posing a dynamic hypothesis, testing it out in a computer model, and 
then revising the dynamic hypothesis until it can do a good job of 
reproducing the real situation and for what appear to be the right 
reasons, and resulting in plausible behavior when extended into the 
future, and even when tested under extreme conditions.

> The other case is the one you mention: creating a new feedback 
> loop(s). But even a new loop serves to push on existing nodes. 
> Otherwise there is no way to attach it to the system. This is why I 
> tend to think in terms of HLPs as being specific nodes.

There is a big difference, both in a model and in real life, between 
creating a new feedback loop and manipulating an existing lever.  
Creating a new feedback loop means designing and implementing a whole 
new way of doing things, which takes a lot more work and creativity than 
tweaking what's already there or already well understood.

- Jack Homer
Posted by  "Jack Homer" <jhomer at comcast.net>
posting date  Wed, 12 Mar 2008 09:36:04 -0400