Ventana software support forum

Jay, I think you are right that SD has grown sometimes faster and sometimes slower. And my perception is also that the last decade has seen a rapid increase in interest. (As good SDers, of course its hard for us to say whether this is due to an increase in the **rate** of expansion or just an increa...

Jack Homer describes a very interesting project in SD0091 that promises to shed light on this discussion. Just to check my understanding, Jack: Youre saying that the stochastic nature of the problem means that you have to track individuals (as opposed to central tendencies of populations) and that t...

This responds to John Wolfendens posting # SD0086. John wants a succinct way of stating system dynamics advantages. John, you seem to be looking for the advantage of the system dynamics approach in terms of the integration method. There are a number of integration methods designed to permit a digita...

Shaun Tang in SD0087 offers further clarification of the discrete vs. continuous discussion. I think that Shauns emphasising that discrete event has variable-length intervals between computations is important and clarifying, because this is what separates discrete event simulations from discrete-tim...

Gene Bellinger in SD0046 briefly and lucidly contrasted discrete-event with continuous-time modeling. Gene, would you also agree that: All modeling approaches need to simplify. One of the key approaches in system dynamics modeling is to aggregate separate "elements" together (e.g. aggregat...

George Richardson writes in response to biology modeling discussion: [I think it is relevant to mention that one approach J.S. Bach used to learn to write music was to copy the manuscripts of the masters who preceeded him.] George would no doubt concur that this is relevant also to learning (and con...

Greg Scholl asks "Since the methodology [of system dynamics] strives to build formal, descriptive structures, shouldnt those structures behave at least similarly to the real-world phenomena they describe? " Absolutely. Indeed the reason to build a model of a structure is to figure out how ...

If you want to model the supply chain, you might take a look at some of the Molecules. In particular, look at Purchasing, Inventory ordering, Capacity ordering, and Inventory backlog shipping. I believe you will see these molecules being used in the Lyneis citation that Michael Radzicki provided. An...

I think that Khalid phrases the issue just right. The goal is

"a rigorous fit of the model to the problem addressed"

Jim Hines
jimhines@interserv.com

Nelson Repenning in SD0014 quotes me as "If I had the full argument in all its richness and subtlty to begin with, I wouldnt need the model at all." and then responds: "I disagree. A major premise of the SD method is that the human minds ability to accurately infer the dynamics of a h...

Bob Powel writes "Mack Alford (Alford Associates) showed a flow diagram of a software development project that contained the basic concepts of the software development waterfall life cycle model. It was from the 1950s and on the SAGE (Semi-Automatic Ground Environment) air defense system develo...

Gene Bellinger writes: "Is there a set of criteria one can employ to determine whether a discrete or a continuous simulation is most appropriate?" Im not exactly sure what you mean by "discrete simulation". If you mean discrete event simulation, then I have the following rather i...

On the side of even fewer bells and whistles: I lead a management team through a pricing excercise in Vensims gaming mode -- no interface, just the model where decisions can be entered every so often. The model and output were projected for all to see; the game was played twice -- once with an aggre...

Tom Fiddaman as usual made a thoughtful contribution to the discussion on variable structure as have a number of others. I would like to comment on Toms suggestion that incorporating genetic programming is not different in principle from regular SD modeling. There are two ways that GPs can be used i...

Will Glass-Husain wrote: > These (multi-player) simulations are powerful highly > engaging experiences, raising many general systems/strategy issues as well > as industry-specific ones. Will, Could you list some of the issues? (I dont want to put you on the spot. Im genuinely interested). Jim Hines ...

Regarding CLD vs. S&F diagrams: The two kinds of diagrams emphasize different things. Causal loops emphasize feedback; stock and flows emphasize, well, stocks and flows. Both feedback and the process of integration create dynamics (i.e. change the shape of an input); and consequently both views ...

Fabian, John Morecroft and I wrote a paper some years ago looking at how one policy for dividing up sales/marketing resources could cause oscillations in product sales (and escalating marketing costs). The paper was based on some work that John Morecroft had done for a large telecommunications compa...

Id say that Tom Fiddamans response to Paige sets the standard for
responding to queries.

Jim Hines
jimhines@interserv.com

David, You "agree strongly" with John that whenever model output and data do not agree alarm bells should go off. But, say that I wish to explore how inventory policies might cause oscillations in the economy. Should I be alarmed that my model does not exhibit a growth mode? Regards, Jim H...

I liked Fred Nickols comments very much and think that they shed some light on the source of the current debate on data and models. People who recommend a course of action and rely in large measure on model output to support the argument, often pay a lot of attention to data. Fitting a model to data...

Benny Budiman in SD0222 offers a clear description of feedback and feedforward in engineering. He concludes by saying: "IMO, feedbacks in SD are actually causal links that include both feedback and feedforward mode of "controlling" the system." Benny is exactly right. The use of ...

We were talking about why companies dont experiment: Experimentation in the hard sciences is meant to test or explore a theory. One reason that experimentation is rare in business is that theories relevant to companies are very rare. System dynamics has something to offer here. Jim Hines JimHines@in...

I have experience in modeling pricing strategies. If you havent already, you might look at the Molecules for pricing and market share. There will be a new version of the molecules up in a few days with about 80 pages of documentation, so its probably best to wait if you havent down-loaded them yet. ...

Bill Cutler in SD0184 writes "As somewhat of an outsider, I guess I dont appreciate why there is so much concern over the question of discrete vs. continuous modeling techniques, and limiting the techniques that can be called SD." I dont think people want to limit techniques. I do think a ...

Concerning Bill Cutlers Posting #SD0185. After offering a terrific example of how model-based insight moved into practice, Bill notes that its "not exactly system dynamics". Id just like to mention that my experience with similar situations involving system dynamics strongly supports what ...