Search found 98 matches

by WayneZhou2009
Tue Oct 08, 2019 7:51 pm
Forum: Vensim
Topic: Probabilistic system dynamic model
Replies: 1
Views: 107

Probabilistic system dynamic model

Hi Tom, Hope you have been doing great. I have a question about using the term "probabilistic" in the context of system dynamics. For a given system dynamics model, suppose its structure is considered deterministic, and its parameters are considered random. After using empirical data to estimate its...
by WayneZhou2009
Thu Jul 04, 2019 8:17 am
Forum: Vensim
Topic: Citing Vensim in academic papers
Replies: 1
Views: 164

Citing Vensim in academic papers

Hi, for academic papers, what is the recommended way of citing Vensim?

Thank you very much.
by WayneZhou2009
Tue Feb 26, 2019 7:34 pm
Forum: Vensim
Topic: Extract a value at a specified time from a stock
Replies: 1
Views: 253

Extract a value at a specified time from a stock

Hi, Is it possible to extract a value at a specified time from a stock? For example, my stock has the following values: Year 2000, stock = 0 Year 2001, stock = 1 Year 2002, stock = 2 Year 2003, stock = 3 ... Year 2029, stock = 29 Year 2030, stock = 30 Is there a straightforward way to extract a valu...
by WayneZhou2009
Tue Nov 06, 2018 6:48 pm
Forum: Vensim
Topic: Use File and built-in distributions for sensitivity
Replies: 1
Views: 440

Use File and built-in distributions for sensitivity

Hi, is it possible to use both File and the built-in distribution for one sensitivity analysis? For example, a user-defined file is used for parameters A and B, and the built-in Random Normal and Random Uniform are used for parameters C and D. Based on this setup, run sensitivity simulation. This do...
by WayneZhou2009
Sat Nov 03, 2018 3:52 pm
Forum: Vensim
Topic: Aging chain with items taken out, on hold and put back
Replies: 20
Views: 2168

Re: Aging chain with items taken out, on hold and put back

Hi JJ, it is still something exploratory. I agree that it is difficult to choose between them, but it is great to have these options available.
by WayneZhou2009
Sat Nov 03, 2018 3:50 pm
Forum: Vensim
Topic: Aging chain with items taken out, on hold and put back
Replies: 20
Views: 2168

Re: Aging chain with items taken out, on hold and put back

Hi Tom, thank you so much for setting up these models, which provide very helpful insights into their inherent dynamics and the emergent outcomes.
by WayneZhou2009
Fri Nov 02, 2018 9:05 pm
Forum: Vensim
Topic: Aging chain with items taken out, on hold and put back
Replies: 20
Views: 2168

Re: Aging chain with items taken out, on hold and put back

To reflect the high variance of the post-retrofit period ("on-hold" period), would the original first-order delay treatment in the picture above be a reasonable approximation?
by WayneZhou2009
Fri Nov 02, 2018 9:03 pm
Forum: Vensim
Topic: Aging chain with items taken out, on hold and put back
Replies: 20
Views: 2168

Re: Aging chain with items taken out, on hold and put back

Thanks Tom for your insight. So what would be a reasonable simplification here if the "on-hold" segment of the model is still needed?
by WayneZhou2009
Fri Nov 02, 2018 8:34 pm
Forum: Vensim
Topic: Aging chain with items taken out, on hold and put back
Replies: 20
Views: 2168

Re: Aging chain with items taken out, on hold and put back

It may be necessary to go to this level in some cases such as infrastructure development, power plants, etc. Hence the distribution of their ages and properties at yearly level would have relevance from policy-making perspective. I'm still not convinced of the policy relevance of preserving every de...
by WayneZhou2009
Fri Nov 02, 2018 7:42 pm
Forum: Vensim
Topic: Aging chain with items taken out, on hold and put back
Replies: 20
Views: 2168

Re: Aging chain with items taken out, on hold and put back

Hi Tom, I feel that this has to be regarded as a 2-dimension problem. So maybe subscripts [i,j] and VECTOR ELM MAP should be used?
tomfid wrote:
Fri Nov 02, 2018 2:41 pm
You could map the return flow from age x to age x+d of the main stock, right?
by WayneZhou2009
Fri Nov 02, 2018 6:53 pm
Forum: Vensim
Topic: Aging chain with items taken out, on hold and put back
Replies: 20
Views: 2168

Re: Aging chain with items taken out, on hold and put back

Hi JJ, thank you so much for your example model. I was hoping to use 2 subscripts but was not able to figure it out. Your approach is enlightening. I have not fully understood the logic behind your model, so I am still studying it carefully. For now, a quick question please: does this model allow wi...
by WayneZhou2009
Fri Nov 02, 2018 1:44 pm
Forum: Vensim
Topic: Aging chain with items taken out, on hold and put back
Replies: 20
Views: 2168

Re: Aging chain with items taken out, on hold and put back

Hi Tom, thanks for your suggestion. However, I do not think that solves the problem, because when those items are being on hold for the required period, they shall keep aging as usual rather than being "frozen". If an infinite-order delay is used, the outcome is that a 5-day-old item entering the de...
by WayneZhou2009
Thu Nov 01, 2018 8:27 pm
Forum: Vensim
Topic: Aging chain with items taken out, on hold and put back
Replies: 20
Views: 2168

Aging chain with items taken out, on hold and put back

Hi Tom, I came across an old post where the question raised was almost exactly the one I am having now and you gave a very enlightening solution using arrays. I have attached it here for easy reference. And the post is: https://www.ventanasystems.co.uk/forum/viewtopic.php?f=2&t=5287 Essentially, the...
by WayneZhou2009
Tue Oct 30, 2018 4:38 pm
Forum: Vensim
Topic: Payoff report has no info on Goodness of Fit etc
Replies: 4
Views: 704

Re: Payoff report has no info on Goodness of Fit etc

Hi Tom, thanks for your reply. I just updated my Vensim Pro to the latest version 7.3.5. But the problem remains. The following is all information contained in my .rep file after running a payoff optimization. Is there a way to show the full information, e.g. Goodness of Fit, etc? The payoff is -20....
by WayneZhou2009
Mon Oct 29, 2018 3:54 pm
Forum: Vensim
Topic: Payoff report has no info on Goodness of Fit etc
Replies: 4
Views: 704

Re: Payoff report has no info on Goodness of Fit etc

Hi,
Would anyone share some experience in this, please? Many thanks.
by WayneZhou2009
Wed Oct 24, 2018 2:29 am
Forum: Vensim
Topic: Payoff report has no info on Goodness of Fit etc
Replies: 4
Views: 704

Payoff report has no info on Goodness of Fit etc

Hi,

This might be a trivial question - why my payoff reporting file does not contain information on Goodness of Fit, Theil Statistics, and Absolute Errors? It only has information on Metadata & Contributions.

Many thanks.
by WayneZhou2009
Tue Oct 16, 2018 2:59 pm
Forum: Vensim
Topic: Payoff sensitivity MCMC does not vary with payoff value (1.92, 3.84 or 6.63)
Replies: 2
Views: 504

Re: Payoff sensitivity MCMC does not vary with payoff value (1.92, 3.84 or 6.63)

Hi Tom, Thanks for your advice. I meant the samples (datapoints). They are all exactly the same, regardless of the payoff value being set as 1.92, 3,84 or 6.63. However the bounds reported in the sensitive.tab are different, as follows: :COM The base payoff is -7.46857 : SENSITIVITY = PAYOFF_MCMC = ...
by WayneZhou2009
Mon Oct 15, 2018 10:26 pm
Forum: Vensim
Topic: Payoff sensitivity MCMC does not vary with payoff value (1.92, 3.84 or 6.63)
Replies: 2
Views: 504

Payoff sensitivity MCMC does not vary with payoff value (1.92, 3.84 or 6.63)

Hi Tom, I was trying to compare the samples resulting from standalone MCMC and payoff sensitivity MCMC, by following the Reference Manual (MCMC Options). While specifying standalone MCMC as the Optimizer is straightforward, I have encountered a problem with specifying MCMC as a method of payoff sens...
by WayneZhou2009
Wed Oct 10, 2018 4:47 pm
Forum: Vensim
Topic: Integration over a time-invariant variable
Replies: 0
Views: 610

Integration over a time-invariant variable

Hi, Is there any way to do integration over a variable that is not related to TIME STEP, i.e. time-invariant? For example, P(mu) is a probability density function of mu where mu is the time-invariant variable which has a predefined range, say, 1 to 10. Now I want to integrate variable A over the dis...
by WayneZhou2009
Fri Oct 05, 2018 6:44 pm
Forum: Vensim
Topic: Mean of errors not close to 0 after optimization
Replies: 5
Views: 722

Re: Mean of errors not close to 0 after optimization

Hi Tom, No, it is the model without the priors. Only MLE, no Bayesian. I have reflected on this issue a bit more. Perhaps I was fundamentally mistaken by expecting the mean of the error to be 0. This case is not linear, and hence in general the mean of error would not necessarily be close to zero? I...
by WayneZhou2009
Thu Oct 04, 2018 9:25 pm
Forum: Vensim
Topic: Using :RAW: for missing data and impact on calibration
Replies: 1
Views: 429

Using :RAW: for missing data and impact on calibration

Hi, I want to ask a question about good practice in handling missing data points in Excel data imported into Vensim, in the context of calibration. Suppose the following is what I want to calibrate: Time 1 2 3 4 5 6 7 8 9 10 Excel data 5 6 -- 5.5 7.3 -- -- 4.5 7.8 9.5 Model 5.3 5.7 5.2 6.0 6.9 3.8 6...
by WayneZhou2009
Thu Oct 04, 2018 9:19 pm
Forum: Vensim
Topic: Mean of errors not close to 0 after optimization
Replies: 5
Views: 722

Re: Mean of errors not close to 0 after optimization

Hi Tom, thanks for your reply. Yes, it is weighted error. The weight is the standard deviation - calculated from the optimization and then plugged back into the model to replace the original weight of 1. The resultant log likelihood is very close to half of the number of data points, namely, around ...
by WayneZhou2009
Thu Oct 04, 2018 7:12 pm
Forum: Vensim
Topic: Mean of errors not close to 0 after optimization
Replies: 5
Views: 722

Mean of errors not close to 0 after optimization

Hi, Regarding optimization through MLE (fitting model to data), what if, after getting the true log likelihood by using the standard deviation of error term as the proper weight, it is found that the mean value of the error term is not very close to 0? Is it a violation of the fundamental assumption...
by WayneZhou2009
Thu Oct 04, 2018 1:52 pm
Forum: Vensim
Topic: Star (*) sign in optimization result
Replies: 3
Views: 589

Re: Star (*) sign in optimization result

Hi Tom, thanks, however I cannot understand what you meant by "value changed on the last pass through the parameters". Please could you elaborate a bit further? Many thanks.
by WayneZhou2009
Tue Oct 02, 2018 10:52 pm
Forum: Vensim
Topic: Star (*) sign in optimization result
Replies: 3
Views: 589

Star (*) sign in optimization result

Hi Tom, Sometimes the optimization result comes with a star (*) ahead of a parameter being optimized. Something like the following: Maximum payoff found at: *Inventory of products = 5e+007 What does the star (*) mean? I remember there are some old posts in which people asked the same question, howev...