REPLY Open Source Simulation Software for SD (SD6797)

[SDMAIL richard dudley]

Posted by  "richard dudley" <richard.dudley at attglobal.net>

I think that Bill's answer clearly indicates why "free" software has not yet
caught on in a larger community.  He mentions about five different software
packages that he uses to produce a final product -- and most of these
require some more detailed level of knowledge about computer _programming_
then many of us have. 

One of the things I find exceptionally good about the standard system
dynamics software packages is that _modeling_ is separated from
_programming_.   I think this is very important for most of us, not only
because we don't want to do programming, but because modeling and
programming are so very different.  A good system dynamics modeler can do
the best work when freed from programming.  For me, at least, I am free to
think about model structure rather than computer code.  I am sure we have
all seen system dynamics models with long strings of embedded if then else
statements.  I always imagine this is an indicator that a  programmer,
rather than a modeler, created this model. 

Stefano really brings up two issues.  One having to do with freely available
software, and the other having to do with inter-operability or open access
code so that models from one software can be used with another.  There has
been a some effort toward this latter idea in the past.  

In reality, at least based on my limited experience, system dynamics seems
to be more inter-operable in a general sense then other modeling paradigms.
To me this is a great strength that has not been widely 'advertised': -  the
stock flow modeling paradigm is fairly standard across software and applies
to any problem being modeled, not just to a specific field.  I can see and
understand a Powersim, or Stella, or Vensim model much more easily than I
could a program written in C or Fortran, for example.

During the review of models, either academic or otherwise, this
standardization can be a huge benefit IF the software makers ensure that, at
a minimum,  they provide fully functional versions which allow one to read,
view the details of, do what if analysis of, and make temporary alterations
to, a model under review.  

Would I like a fully functional, free, user friendly system dynamics
software?  Of course!  But I have also come to appreciate the amount of work
and talent that has gone into the design of the existing packages.
Presumably these people need to be paid for doing this. 

Richard
Posted by  "richard dudley" <richard.dudley at attglobal.net>
posting date  Sat, 8 Mar 2008 09:19:21 +0700