REPLY Definition of root cause (SD6828)

[SDMAIL Jean-Jacques Laublé]

Posted by  Jean-Jacques Laublé <jean-jacques.lauble at wanadoo.fr>

Hi Jack.

This is a very theoretical discussion, and I try to apply SD to very
concrete problems.
I think that to illustrate your ideas and to prove that they can have a
practical application, it would be a good idea to show an example even very simple
with a model (we must not forget that SD has a lot to do about modelling)
that shows how to use your ideas and what is the added value of doing it.
Building such a model would give a basis for this discussion.
Otherwise if one cannot build a model illustrating the point, this
discussion has no interest for me. I precise: for me, as it may have an interest for other
people.

I agree that a model is always an hypothesis but what I was writing is about
how models are built. The classical method works with reference modes and
tries to imagine hypothesis that explains these behaviour. Coyle does not
base the way of building models on these reference modes, arguing that it
does not make sense to use reference modes at the same time as basis for the
construction of models and as basis for their validation unless it is only
to verify that the first work has been correctly done and that there is no
error in the process. But it cannot be used twice once for the conceptual
building and twice to verify the same concept.

I prefer to work without explicit reference modes during the building
process, even if I can have them in the back of my mind, because reference
modes are often not available in practical work.
You may have completely new situations where reference modes can be
misleading and it is better to think about diverse scenarios. You may miss
reference modes because of unavailability of data or too high costs to get
them.

If I have reference modes and it is of course a great help, I will try to
use them as a validation tool.

About closed models, Coyle studies both closed and open models.
My problems are mostly and unfortunately open, although I recognize that
working with closed models is better for reasons of simplicity. But I must
adapt my models to both endogenous influence and exogenous ones.

I recognise too that this the way I think presently and that it may change
in the future.
I am not sure that Jay Forrester was really thinking about using dynamic
hypothesis when writing about the necessity of a model to reproduce reality.
Jay knows certainly better than us what he meant.
About your root causes it looks very much like boundary extensions.
I replace the heap of tortoises by a box in a box in a box in a box etc.

You write :
<This is because the field still clings to phrases
<like "dynamic hypothesis" of what is causing the problem instead of root
<cause and feels that if a model can reproduce the symptoms, then it must
<contain the root cause.

I do not think that anybody pretends that reproducing the symptoms does
guarantee anything.

The notion of dynamic hypothesis is not part of the SD paradigm but belongs
to the most known and taught method of building models.

Some authors do not use dynamic hypotheses while respecting strictly the SD
paradigm. For instance R.G. Coyle never mentions dynamic hypotheses and
builds his models only from the principle of causality, not necessarily root
one because of the 'heap of tortoises'. He does not use either reference
modes to build models but only as a way of comparing reality to the model
behaviour once it is built.

And I personally prefer much more this way of building models than the
official one.

Regards.
Jean-Jacques Laublé Eurli Allocar
Strasbourg France.
Posted by  Jean-Jacques Laublé <jean-jacques.lauble at wanadoo.fr>
posting date  Thu, 20 Mar 2008 10:38:19 +0100