Search found 3048 matches

by tomfid
Wed May 13, 2020 4:23 pm
Forum: Vensim
Topic: Delay conveyor sensitive to time step?
Replies: 3
Views: 401

Re: Delay conveyor sensitive to time step?

If you want a continuous time step and visibility of the detailed cohorts so that you can have age-specific loss rates, I'd seriously consider dumping the aging chain in favor of a strategy more like the dynamic cohorts in Ventity. That would entail having the array dimensions represent model years ...
by tomfid
Wed May 13, 2020 4:17 pm
Forum: Vensim
Topic: Delay conveyor sensitive to time step?
Replies: 3
Views: 401

Re: Delay conveyor sensitive to time step?

I think this is not a valid use of DELAY CONVEYOR with leakage, because the dynamics of the internal state in the delay function in the aging flow don't match the dynamics of the vehicle fleet stock. You have scrap = stock* rate, but from the Help: When leak is nonzero than the amount of material is...
by tomfid
Wed May 13, 2020 2:19 pm
Forum: Vensim
Topic: Random Function
Replies: 21
Views: 1165

Re: Random Function

One argument for a more physical model of failure is that there are probably two causes: - passage of time - e.g. degradation of rubber seals from exposure to oxygen - wear and tear from use I would guess that the 2nd is most of the problem, so that a failure rate expressed as 1/year is making assum...
by tomfid
Wed May 13, 2020 2:03 pm
Forum: Vensim
Topic: Random Function
Replies: 21
Views: 1165

Re: Random Function

For your purpose, I'm not sure you need it. You can use the Weibull hazard as a function of time, as long as feedback isn't important. Otherwise, I think you'd have to abandon the Weibull distribution in favor of a mixture of exponentials. Or maybe you can find the math behind the Weibull - I haven'...
by tomfid
Tue May 12, 2020 8:02 pm
Forum: Vensim
Topic: Random Function
Replies: 21
Views: 1165

Re: Random Function

You could modify the top structure to use the Weibull hazard rate rather than constant. However, that's not very satisfactory as a dynamic model, because that makes the rate a function of time, and what's really wanted is a function of state. The Weibull distribution is handy for its flexibility, bu...
by tomfid
Tue May 12, 2020 7:59 pm
Forum: Vensim
Topic: Random Function
Replies: 21
Views: 1165

Re: Random Function

Ah ... makes sense. Here's a toy model. hazard ex 1.mdl The top structure is a constant hazard rate yielding an exponential (= Erlang 1 = Gamma 1) failure distribution. You could add 2nd stage to get Erlang 2, etc. The bottom structure is a little more physical, with an unobserved state that degrade...
by tomfid
Mon May 11, 2020 6:02 pm
Forum: Vensim
Topic: Random Function
Replies: 21
Views: 1165

Re: Random Function

I'm still not clear on what we're talking about here. Do we have a single device with 4 failure modes? Or a single population subject to multiple hazards? Are these hazards rare events? Do the occur discretely or continuously? A plain-language description of what the model is trying to do is needed.
by tomfid
Mon May 11, 2020 3:12 pm
Forum: Vensim
Topic: Random Function
Replies: 21
Views: 1165

Re: Random Function

I'm still not clear on what this is trying to do. The units on FLOW HARM are probability, which means the units on HARM are probability*time, which is hard to interpret.

RANDOM EXPONENTIAL(0, 1e+12 , 0 , Frequency CF18*Time , 0 ) implies a distribution with a mean that is increasing with time.
by tomfid
Mon May 11, 2020 3:04 pm
Forum: Vensim
Topic: Random Function
Replies: 21
Views: 1165

Re: Random Function

There is no limit field. The equation logic itself has to implement the logic. You can always constrain things with something like MIN(1,MAX(0,expression)), but if your pdf doesn't integrate to [0,1] you probably have a problem with the expression itself.
by tomfid
Mon May 11, 2020 12:41 am
Forum: Vensim
Topic: Random Function
Replies: 21
Views: 1165

Re: Random Function

Ahh... The Min and Max fields in the equation editor are not hard limits; they only generate warnings at runtime if exceeded. That's what you see in your second image. They also control bounds on synthesim sliders, if the variable is a constant. If you want a limit, you have to implement it in the e...
by tomfid
Sun May 10, 2020 5:29 pm
Forum: Vensim
Topic: Random Function
Replies: 21
Views: 1165

Re: Random Function

You might check the sample models attached to the various help topics, if you haven't already. The point of the min/max parameters is to permit truncation of the distributions. For example, calling RANDOM NORMAL(-3,3,0,1,0) truncates the Normal distribution at +/- 3 standard deviations. If you use: ...
by tomfid
Sat May 09, 2020 11:58 pm
Forum: Vensim
Topic: Confusion about the correlation time unit in RANDOM PINK NOISE funtion
Replies: 5
Views: 538

Re: Confusion about the correlation time unit in RANDOM PINK NOISE funtion

Correct. Behavior changes are expected with random functions, and not a bad thing, except to the extent that it makes analysis harder. If you use the "1/3" rule, be sure to consider that the higher-order delay functions like SMOOTH3 and DELAY3 have a time constant that is (delay time)/(delay order)....
by tomfid
Sat May 09, 2020 9:24 pm
Forum: Vensim
Topic: What improvements would you like to see in Vensim?
Replies: 109
Views: 65452

Re: What improvements would you like to see in Vensim?

@EwoutH a new graphics system is in internal testing already, so this should be improved soon. @jpain3 - good ideas.
by tomfid
Sat May 09, 2020 9:21 pm
Forum: System Dynamics Discussion
Topic: what method of generating random variable is correct in this problem?
Replies: 2
Views: 1371

Re: what method of generating random variable is correct in this problem?

This particular subforum doesn't get many eyeballs, I think.

I don't see a strong reason for pink noise here, because there's no obvious way that patients' characteristics would be correlated over time. Seems more like a case for white noise (i.e. RANDOM NORMAL or similar).
by tomfid
Sat May 09, 2020 4:40 pm
Forum: Vensim
Topic: The subscripts problem in aging structure
Replies: 4
Views: 473

Re: The subscripts problem in aging structure

Your current subscript implies that there are 9 values for Installed Capacity Vintage 1 - why would that be the case?
by tomfid
Sat May 09, 2020 4:37 pm
Forum: Vensim
Topic: The subscripts problem in aging structure
Replies: 4
Views: 473

Re: The subscripts problem in aging structure

You don't want subscripts for that; you want a coflow structure. Take a look at the coflows of capital and efficiency in https://metasd.com/2010/03/the-energy-t ... e-economy/
by tomfid
Sat May 09, 2020 4:28 pm
Forum: Vensim
Topic: Confusion about the correlation time unit in RANDOM PINK NOISE funtion
Replies: 5
Views: 538

Re: Confusion about the correlation time unit in RANDOM PINK NOISE funtion

The rule of thumb for 1/3 the smallest delay is reasonable, but sometimes not sufficient. Really the rule should be 1/3 the shortest effective time constant, whether it is in a negative or positive loop, and this is not always known. The simplest thing to do is do some testing - lower the time step ...
by tomfid
Fri May 08, 2020 6:18 pm
Forum: Vensim
Topic: The subscripts problem in aging structure
Replies: 4
Views: 473

Re: The subscripts problem in aging structure

It seems like you're doubling up on vintaging here. If you have explicit stocks for vintages, you don't need to repeat that in the subscripts. The subscript range could simply represent wind, solar, etc.
by tomfid
Thu May 07, 2020 8:56 pm
Forum: Vensim
Topic: A question about GET XLS DATA
Replies: 16
Views: 1124

Re: A question about GET XLS DATA

I'm disenchanted with the cloud versions of Excel, which seem to get flakier with each update. I've switched my GET XLS protocol to Direct (on the Settings tab of Tools>Options). That makes GET XLS work like GET DIRECT. The downside is that the file can't be open in Excel. The benefit is that these ...
by tomfid
Thu May 07, 2020 7:44 pm
Forum: Vensim
Topic: A question about GET XLS DATA
Replies: 16
Views: 1124

Re: A question about GET XLS DATA

If it's reporting "does not exist" you probably have a path error. Change the equation to GET XLS DATA('?mydata', 'Sheet1' , '1' , 'B2' ) and it will query you to locate the file when you run the model. (Be sure that it's not open in Excel, because Excel locks the file.)
by tomfid
Thu May 07, 2020 6:26 pm
Forum: Vensim
Topic: A question about GET XLS DATA
Replies: 16
Views: 1124

Re: A question about GET XLS DATA

You could just save it as csv or tab and import that.
by tomfid
Tue Apr 28, 2020 11:39 pm
Forum: Vensim
Topic: Question relating to queues / delays / waiting
Replies: 3
Views: 540

Re: Question relating to queues / delays / waiting

This is doable with stocks and flows as well as with the fancier queue and delay functions. A lot depends on whether you need the detail of integer people or not (i.e. are you building a discrete event simulation, or a continuous SD model). Do discrete beds and people matter, or is this more strateg...
by tomfid
Tue Apr 21, 2020 3:47 am
Forum: Vensim
Topic: math operations on simulated scenario results
Replies: 3
Views: 563

Re: math operations on simulated scenario results

Another option is to use an array dimension to create multiple instances of the model - then you can operate on the array values.
by tomfid
Mon Apr 20, 2020 9:02 pm
Forum: Vensim
Topic: Help with usage of MCMC
Replies: 11
Views: 994

Re: Help with usage of MCMC

It looks like your cooling period is longer than your burnin period, so there's an abrupt drop in temp from 5.77 to 1. I'd try changing the lengths so that doesn't happen, or just turn off the cooling altogether (you may find that you don't need it anymore).
by tomfid
Mon Apr 20, 2020 6:46 pm
Forum: Vensim
Topic: Help with usage of MCMC
Replies: 11
Views: 994

Re: Help with usage of MCMC

I think this has to do with a reset of the stats that happens at the end of the burnin period. Tentatively, the new values may not be right for a while. This is potentially ignorable, but I'd have to see a run to be sure. I guess I'd be most concerned about a drop in the acceptance rate. One thing t...