Ventana software support forum

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> Peter Marks's question is very easy to answer. There are numerous examples in the literature. Suppose there is a variable, X, which, in some circumstances depends on Y, but, in other cases, depends on Z. Also, whichever of th...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> Bill, I think that you are broadly right, though you still imply that the task is to 'sell' SD. Of course, it is a wonderful method when properly used for suitable issues but the mistake, maybe, is to call ourselves System Dy...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> John is absolutely spot on about the vital importance of really knowing about a domain, but that still leaves a difficulty. On a purely personal note, I hope that I have some knowledge, both theoretical and practical, about t...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> Jean-Jacques Lauble wrote: "" I do not know if it is the same in England, but in France you can go to a bookstore and look at many books dealing with strategy for instance, and find no books that use the stock and f...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> Jean-Jacques also questions why the great majority of operations research/management science texts do not mention SD. Those that do often lump it with simulation in general. In other words, SD has been in existence now for pr...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> This is an interesting correspondence though it does not seem to have answered the original question, which was about SD and other modelling approaches. In that context I have always felt that one cannot do good SD without at...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> Many of you will know Mike Kennedy of London South Bank University from his presence at, and participation in, the Society's conferences. Most recently, he was one of the organisers of the Oxford Conference and he played a pi...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> I am afraid that Lukkenaer is confused about time in SD models (he is not alone). Let us suppose that the nature of the system we are dealing with is such that it only makes sense to think in terms of weeks and that, in order...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> George makes an interesting point here. He is right, in a sense, as it has always been the case, going right back to the start of SD, that dt should not be used on the RHS of rate and auxiliary equations, to use the original ...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> There is a very simple solution to this issue, but Bill's question also confounds that with some equations that I can't really follow. His equations might be re-written as: potential_customers=potential_customers-dt*rate_of_w...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> Sorry for this late rely to Bill Braun's query, but I've been struggling with sun, food (and the odd glass of wine) in Cyprus. It was tough, believe me! It is, of course, true that cascading N first order delays will produce ...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> The usual lucid good sense from George Richardson. The lad is showing promise and, in another 10 or 20 years ... Seriously, the 'models are wrong but can be useful' phrase might be used when students are just starting, or as ...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> Bill's first paragraph is correct. The order of a model is the number of stocks it contains. That includes, however, the stocks internal to delays so, a model with two explicit stocks and a third order delay is of fifth order...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> Many of you will know Dr Bob Cavana through his tireless work for the SD Society, and for organising the conference that was held in Wellington, New Zealand. I am delighted to be able to tell you that he has now been elevated...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> A further comment on this question of publishing in journals addressing other domains is the importance of becoming reasonably knowledgeable about the domain. That takes time, but is vital if the paper is not to be rejected o...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> It is certainly worth publishing in journals other than SDR; one reason being that SDR obviously has limited capacity. A better reason is that one gets useful feedback from referees who are not steeped in SD. An even better r...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> I think that there are three extra factors that need to be taken into account in Kim's reasoning. The first is that customers are lost in any case - they die, move away, or are seduced by the competition. That can be represen...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> Related to Hamilton's book is a very interesting volume entitled 'Causal Inference in Non-Experimental Research'; I think that the author's name was Blalock. I am sure that there is at least one valuable PhD in that topic. Re...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> As Alan points out, optimisation has been used in SD for a very long time. I think that the first uses go back to the 1970s. I no longer get System Dynamics Review, so I haven't seen his paper, but I guess that his references...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> Dear All, One easy way to do this is to write X=A/(B+EPS) where EPS is a very small number - 0.001 or less and has the same dimensions as B. There are more sophisticated possibilities but this quick and dirty method has alway...

Posted by ""geoff coyle"" <geoff.coyle@btinternet.com> I don't think that you can usefully compare SD and DES - they are different things for different purposes. In any case, Mike Pidd's textbooks have 'compared' SD and DES for many years past. Regards, Geoff Visiting Professor o...