Search found 209 matches

by gwr
Thu Nov 14, 2019 12:23 pm
Forum: Vensim
Topic: Documentation for Delay Information gives false examples
Replies: 6
Views: 7615

Re: Documentation for Delay Information gives false examples

To make this self-contained: The correct outputs (at least correct with regard to the behavior that should be expected from reading the Delay Information documentation) for the examples given in the documentation are: https://i.stack.imgur.com/AwWGp.png https://i.stack.imgur.com/05cPi.png Interestin...
by gwr
Wed Nov 13, 2019 9:17 pm
Forum: Vensim
Topic: Documentation for Delay Information gives false examples
Replies: 6
Views: 7615

Re: Documentation for Delay Information gives false examples

Why not have both versions? I just decided to have both versions using Modelica (where changes are also instantaneous as events per definition are not consuming time). Let’s assume, that you are looking at stock information and all you can see is the next tick - but you never see what time it is fro...
by gwr
Wed Nov 13, 2019 8:00 pm
Forum: Vensim
Topic: Documentation for Delay Information gives false examples
Replies: 6
Views: 7615

Re: Documentation for Delay Information gives false examples

Thanks, for acknowledging the false behavior, Tom - not everybody does. ;-) Writing information delays with variable delay times is tricky imo: Holding the information we have when the delay time increases (delayed information will then be older than what we already know) makes the implicit assumpti...
by gwr
Wed Nov 13, 2019 6:03 pm
Forum: Vensim
Topic: Documentation for Delay Information gives false examples
Replies: 6
Views: 7615

Documentation for Delay Information gives false examples

Hi, I noted that the online documentation for Delay Information gives the following examples: https://i.stack.imgur.com/UkQWk.png The examples SS1 and SS2 are wrong imo: The delay time should step up at time = 40 and thus the delayed input should be held starting at that point in time until more rec...
by gwr
Tue Aug 04, 2015 9:30 am
Forum: Ventity
Topic: difference between the 64 and 32 version
Replies: 3
Views: 6155

Re: difference between the 64 and 32 version

Hi, I do not totally understand why the 64b architecture should only be linked to model size? After all being able to do calculations 'en bloc' and in memory should help to speed up calculations? What about paralellization - which should be an issue for agent based modeling and sensitivity runs (eg....
by gwr
Tue Aug 04, 2015 8:59 am
Forum: Ventity
Topic: Possible performance issue with Windows Update
Replies: 4
Views: 6756

Re: Possible performance issue with Windows Update

Hi Tom,

does this concern a regular Windows update (eg. for WIndows 7 in my case) or is it related to Windows 10?

Best regards,

Guido
by gwr
Thu May 21, 2015 1:40 pm
Forum: Vensim
Topic: continuous delay
Replies: 6
Views: 7891

Re: continuous delay

Hi, there is a nice article by Margret S. Hamilton in Randers' "Elements of the System Dynamics Method" (1980) about how to estimate Delays. Since exponential delays can be linked to the Erlang-Distribution (via convolution) one can build upon this. I have build a small Mathematica-based t...
by gwr
Thu Mar 05, 2015 5:24 pm
Forum: Vensim
Topic: How to use Markov Chain Monte Carlo in Vensim?
Replies: 33
Views: 26923

Re: How to use Markov Chain Monte Carlo in Vensim?

For completeness: Lecture on MCMC for System Dynamics by N. Osgood Pesentation on MCMC for System Dynamics by N. Osgood with examples 2013 Conference Paper on MCMC by N. Osgood There is also a link to doing sequential Monte Carlo (Particle Filtering) to be found on the YouTube sites above -- albeit ...
by gwr
Thu Dec 04, 2014 5:25 pm
Forum: Vensim
Topic: problems with small numbers
Replies: 6
Views: 6086

Re: problems with small numbers

This seems quite related to numerical problems in estimating LOGIT - models (e.g. logistic regression) and LOGSUMS so maybe some of the links to be found in the following answers on stackoverflow may be of help also:

Avoiding numerical overflow with a logistic loss function
by gwr
Wed Nov 19, 2014 5:23 pm
Forum: Vensim
Topic: basic vensim computation issue 2
Replies: 15
Views: 22193

Re: basic vensim computation issue 2

Hi JJ, totally d'accord - there are lots of more important issues than rounding errors. And yet - it really helps to test whether an option matters to be able to "see what happens if that option is chosen". Why else - for example - are we taught to try different Time Steps and check whethe...
by gwr
Wed Nov 19, 2014 2:32 pm
Forum: Vensim
Topic: basic vensim computation issue 2
Replies: 15
Views: 22193

Re: basic vensim computation issue 2

However, error propagation is not free, so the question is whether the security is worth it. Ideally, we'd have two engines in Vensim - one fast, one secure. I'd guess that a lower bound on the speed penalty for error propagation is 2x. There should indeed be no free-lunch and that is probably why ...
by gwr
Thu Nov 13, 2014 11:28 am
Forum: Vensim
Topic: basic vensim computation issue 2
Replies: 15
Views: 22193

Re: basic vensim computation issue 2

I think we'd need the first few values from the mathematica version at full precision. It may be mathematica that's weird. I would not say that Mathematica' s (or as of now more correctly the Wolfram Language's ) way of doing numerical calulations is "weird" but it certainly is " dif...
by gwr
Wed Nov 12, 2014 7:36 pm
Forum: Vensim
Topic: basic vensim computation issue 2
Replies: 15
Views: 22193

Re: basic vensim computation issue 2

There is nothing to look at for you in particular with regard to the model attached -- I have provided it as a suggestion to other users in the forum. What you might care to look at is the remaining question that I had put forward with regard to the MachinePrecision vs. DoublePrecision issue though....
by gwr
Wed Nov 12, 2014 6:31 pm
Forum: Vensim
Topic: basic vensim computation issue 2
Replies: 15
Views: 22193

Re: basic vensim computation issue 2

Detecting rounding errors is part of model development/debugging. As this web page suggests (http://support.microsoft.com/kb/214118), you need to minimize the effects of floating point arithmetic (things like using ROUND, and IF THEN ELSE ( value >= 1) are good examples). Yes, acknowledged and I am...
by gwr
Wed Nov 12, 2014 5:10 pm
Forum: Vensim
Topic: basic vensim computation issue 2
Replies: 15
Views: 22193

Re: basic vensim computation issue 2

I do not quite understand, why my post which was concerned with the question posted here ( http://www.ventanasystems.co.uk/forum/viewtopic.php?f=2&t=5807 ) has been moved to another thread? But anyhow I have uploaded the model which is in accordance to the OP in that thread: stock = INTEG( inflo...
by gwr
Wed Nov 12, 2014 3:00 pm
Forum: Vensim
Topic: basic vensim computation issue 2
Replies: 15
Views: 22193

basic vensim computation issue 2

Hm, while the issue ( http://www.ventanasystems.co.uk/forum/viewtopic.php?f=2&t=5807 ) seems to be trivial and we usually are "taught" to never check for exact values I do wonder about Vensim's double precision a bit. I have checked the issue of the OP using Mathematica for three cases...
by gwr
Mon Nov 03, 2014 6:19 pm
Forum: Vensim
Topic: computer speed
Replies: 22
Views: 24272

Re: computer speed

Only the multi-context DLL supports this.
Great. And this multi-context DLL is included where or can be obtained how?
by gwr
Mon Nov 03, 2014 4:27 pm
Forum: Vensim
Topic: computer speed
Replies: 22
Views: 24272

Re: computer speed

Great answers to ponder, thank you, Tom! Just for correctness: It should have read "DASE / DOE" (Design and Analysis of Simulation Experiments / Design of Experiments) in my post with DASE being used by Kleijnen having the connotations of DACE. I understood that it is possible to start mul...
by gwr
Mon Nov 03, 2014 12:07 pm
Forum: Vensim
Topic: how to limit the number of simulation when optimizing
Replies: 10
Views: 9567

Re: how to limit the number of simulation when optimizing

Hi JJ,

I see that you are using the optimization control option

Code: Select all

:SIMS_MAX = 10
I fail to locate this in the reference manual - would you have a link here?

Regards,
Guido
by gwr
Mon Nov 03, 2014 11:56 am
Forum: Vensim
Topic: how to limit the number of simulation when optimizing
Replies: 10
Views: 9567

Re: how to limit the number of simulation when optimizing

I upload the file with a mdl extension because the voc is not allowed.
That is a bit unfortunate admittedly but I found you can always upload a zip-archive which might also be a good idea in itself?

Regards,
Guido
by gwr
Mon Nov 03, 2014 11:53 am
Forum: Vensim
Topic: computer speed
Replies: 22
Views: 24272

Re: computer speed

So if that is what you are saying, then DSS can compile models and it does exactly this. Yes, that is essentially what compiling should be all about. Once again a great answer even though what I was trying to say - making some effort in doing so - failed to be reasonably clear for some reason. Albe...
by gwr
Mon Nov 03, 2014 10:26 am
Forum: Vensim
Topic: computer speed
Replies: 22
Views: 24272

Re: computer speed

That is an interesting suggestion with regard to renting virtual machines in the cloud. With regard to what might speed up simulation I would like to know more about how things are done internally in Vensim with regard to sensitivity runs, synthesim or optimization runs: For example I noted that in ...
by gwr
Sun Nov 02, 2014 10:28 pm
Forum: Vensim
Topic: Are there inconsistencies in the Vensim DLL ?
Replies: 6
Views: 7042

Re: Are there inconsistencies in the Vensim DLL ?

Great to hear and thanks for the advice so far which should help to circumvent difficulties til then.

Best,
Guido
by gwr
Sun Nov 02, 2014 10:26 pm
Forum: Vensim
Topic: computer speed
Replies: 22
Views: 24272

Re: computer speed

The advance in hardware performance is argueably only part of the reason software has gotten faster, the other side to it being algorithmic advances . To my knowledge, Ventity for example, will follow modern solver strategies and use a variable time step, loosely speaking making "bigger steps&q...
by gwr
Thu Oct 30, 2014 9:16 am
Forum: Vensim
Topic: Are there inconsistencies in the Vensim DLL ?
Replies: 6
Views: 7042

Re: Are there inconsistencies in the Vensim DLL ?

Hi Tom, I am having trouble following your advice: I have checked that the Options for Macro variables in the Tools-Menu are set to "hide" and I have checked the settings to be included when publishing the .vpm-file. Unfortunately that does not alleviate the problem. The follwing also did ...