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Folks, Some time ago I put out an announcement of a stream of qualitative SD for Oxford. That has produced a good deal of reaction and we are well on the way to a very successful stream covering a wide variety of issues. You were asked to send abstracts to me, and its still not too late to submit yo...

Yaman RIGHTLY reminds us of what, and Jay Forrester pointed this out well over 40 years ago, is the fundamental test - that the models defence rests on justifying its equations, not on its comparison to observed data. To be sure (and this was not Kims original question) it is comforting if the simul...

Dear SD colleagues and friends, I am pleased to tell you that my new book, Practical Strategy: Structured Tools and Techniques, was published by Pearson Education in November 2003. Unlike my previous work, this book has very little about SD so why should you be interested? The reason is that the boo...

The growth in qualitative system dynamics, QSD, in which only a diagram is built and there is no simulation, has been considerable over the years. In particular recent work on the use of structural analysis to replace detailed, simulation-based dynamic investigation has shown how powerful this is fo...

As usual, Jay has hit the nail on the head. Whether one uses one simulation paradigm or another depends on the nature of the problem, the purpose of the analysis and, perhaps above all, on the nature of the question to be answered. I tried to make the same points in my earlier email about distributi...

1 This is a common class of problem that arises in many domains - the workload in a machine shop is but one of many cases. It is usually addressed, as you have done, by discrete event simulation (DES) using any one of a number of DES languages. (CSL was a good one but Im not sure what is now availab...

This sounds like a fascinating question but Id hate to rush into it without
finding out more about the domain knowledge. Can your friend point you to
what Id guess would be an extensive literature?

Geoff
From: "geoff coyle" <geoff.coyle@btinternet.com>

John rightly cites the volume of work on models (something that I mentioned in my first query). Its certainly an impressive collection, ranging over the last 20 years but it does not, as far as I can tell, address the issue of empirical evidence for the waves existence. Geoff From: "geoff coyle...

Brenda refers to the macroeconomic variables, but surely we also need to talk about long waves (if they exist) in more general social/political/technological variables. I do though, support her view that we need some kind of consensus on what a wave would have to include before we could start to loo...

Folks, This could well turn out to be a very interesting correspondence. Bill posed a fascinating question but could find only two mentions of Kondratieff on Google, suggesting that the long wave is not a big issue with economists. Jay (Forrest) is comfortable with back to basics. The trouble with t...

This kind of question is interesting but, all too often, the questioner does not make it clear why he/she wants to do what they are asking about. That makes it hard to send a sensible and helpful reply. Id urge people to be more precise. [ Hosts Note: Geoff is absolutely right. If you want to post a...

Folks, This correspondence about the Rev Dr Thomas Bayes, the Mujahidin, and Afghanistan reminds me of the USS Maine (bear with me, there will be two points). Point 1 (a little light-hearted). The sinking of this battleship in Havana harbour, apparently as a result of a terrorist act, led to the Spa...

George makes a good point. One of the uses of expert systems is to archive the knowledge of skilled and experienced people. There is, as he says, a need for some really good research into how experienced modellers (and not only in SD) go about their business. How, then, do novices learn the same ski...

This strikes me as being a bit dodgy. Lets look at the park lighting first and imagine a scale from 0 to 1. So, 0 means no lighting at all, so thats clear (well, dark actually, at any rate at night). At the top end , 1 must mean perfect, but what does that actually mean? What is perfect to one perso...

Joel, I stand corrected! Well done, though I havent seen dimensions/units even mentioned in the index of any OR text (though I havent reviewed all of them). One re-order formula I know of, which is in regular use by a large organisation, is dimensionally invalid! When I pointed that out, they just l...

What an interesting problem. How does it fit with Baumols Economic Dynamics, or is that old hat? Id raise three issues: First, and maybe most important, what is the purpose of the model? What well-chosen questions is it designed to answer? Second, how have you ensured that it is dimensionally consis...

Bruce wrote: > > Next why do you feel like that it would be an inappropriate time for SD > work on terorism to be published? You are correct, everyone can read, but > do feel that the work currently being published (and read) on terrorism is > the most appropriate? Wouldnt now be an ideal time to of...

Jim, as usual, adds interesting ideas to a debate that seems to have got out of all proportion. > Also: Geoff, what are the three permissible uses of dt? Non-negativity, as discussed; introducing quantity via PULSE (depends on the s/w); resetting things such as accounts to zero at the end of a year ...

Raymond, Surely youd treat the stock as the quantity of water in the tanks (probably two separate stocks). The depth of water in a tank is then an auxiliary calculated from the quantity and the size of the tank. The head of water between the tanks is another auxiliary, representing the pressure diff...

Bill asks if treating negative stock explicitly as a backlog is a heuristic or a firm rule and refers to his financial problems (donations can be sent to me and Ill pass them on, less the usual commission). Id suggest that it depends on whether or not behaviour changes. Someone whose bank balance is...

Date: Wed, 17 Jul 2002 09:59:53 +0100 MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Jim Hines says: > First is the question of where the word "widget" comes from. According > to the American Heritage Dictionary, the word "wi...

Christopher is right, the use of /DT is only one way of representing a constraint. It corresponds to a policy of deliver until we run out of stock. An alternative is a policy of slow deliveries (or whatever) as we get closer to stock out. The latter would be better handled by a table function though...

Rod MacDonald asks: > I was wondering what information exists on the listserve about folks > undertaking modeling projects that deal with terrorism in any of its This is a subject on which work has been published, but some time ago. However, I am not at all sure that it would be a good idea to publi...

Jim makes some good and helpful points, and corrects my error that the list is society sponsored. Other respondents have misread my suggestion and reacted less sensibly. Of course, no one - least of all me - wants to limit discussion to Society members or to turn away enquiries. All I said was that ...

Not sure I really agree with Yaman. SD treats all entities as though they are physical quantities (even soft variables such as Consumer Satisfaction) and physical systems usually cannot have negative quantities (there are exceptions - a negative STOCK might be a backlog). So, if something that shoul...