Search found 2936 matches

by tomfid
Sun May 24, 2020 3:07 am
Forum: Vensim
Topic: Calibrating various scenarios beside the baseline
Replies: 3
Views: 19

Re: Calibrating various scenarios beside the baseline

In that case, I think you'd want to recalibrate each of those.
by tomfid
Sun May 24, 2020 1:17 am
Forum: Vensim
Topic: Calibrating various scenarios beside the baseline
Replies: 3
Views: 19

Re: Calibrating various scenarios beside the baseline

I think the answer depends on whether the scenarios differ only in terms of future policy, or also past variables that would affect the fit to history. Normally I would think of there being a single baseline, with policy scenarios varying around that. The way to implement this is to calibrate the mo...
by tomfid
Sat May 23, 2020 12:14 am
Forum: Vensim
Topic: Low optimisation speed
Replies: 11
Views: 388

Re: Low optimisation speed

I just ran a test on a Ryzen 9 3900x. According to benchmarks, it should be faster than my dual Xeon workstation. But it was 4x slower. I'll investigate further. At the moment, the best bang for the buck is probably a fast i7 or i9. Core count will matter in the forthcoming parallel version, but oth...
by tomfid
Fri May 22, 2020 4:04 pm
Forum: Vensim
Topic: custom sensititity histograms
Replies: 1
Views: 11

Re: custom sensititity histograms

It should be, but looking at the docs, it's not in the language.
https://www.vensim.com/documentation/24010.htm
Will investigate further and put it on the todo list.
by tomfid
Wed May 20, 2020 5:58 pm
Forum: Vensim
Topic: Syntax highlighting? a) in Vensim, b) text exported from Vensim
Replies: 6
Views: 24

Re: Syntax highlighting? a) in Vensim, b) text exported from Vensim

Notepad++ has definable highlighting. Not sure how helpful it would be, but seems interesting to try.
by tomfid
Wed May 20, 2020 3:43 pm
Forum: Vensim
Topic: Using data from the previous time // Vensim PLE
Replies: 10
Views: 46

Re: Using data from the previous time // Vensim PLE

Also, all parameters should be made into explicit variables with assigned units. This includes dimensionless parameters, because they should be exposed to sensitivity analysis, documented and consistent if used more than once. The only exception is things that are invariant or definitional, like 1 o...
by tomfid
Wed May 20, 2020 3:39 pm
Forum: Vensim
Topic: Using data from the previous time // Vensim PLE
Replies: 10
Views: 46

Re: Using data from the previous time // Vensim PLE

You've initialized "previous speed" to 0. Initialize it to "speed" or 40 and it should work. However... it won't be a good dynamic model. I'm guessing that this is a replication of some kind of regression study, in which someone took the underlying physics of the situation, differentiated and discre...
by tomfid
Wed May 20, 2020 3:24 pm
Forum: Vensim
Topic: Setting global default arrow sketch behavior
Replies: 2
Views: 17

Re: Setting global default arrow sketch behavior

It's not part of the defaults, but you can use Ctl-A or Edit>Or Select...>Information Arrows to select all arrows, then change them together using the bottom toolbar options.
by tomfid
Wed May 20, 2020 12:58 am
Forum: Vensim
Topic: Any built-in options to relocate and re-layout the parameters places?
Replies: 1
Views: 16

Re: Any built-in options to relocate and re-layout the parameters places?

Unfortunately not, thought the forthcoming new graphics engine might make that practical to implement.
by tomfid
Tue May 19, 2020 2:42 pm
Forum: Vensim
Topic: Using data from the previous time // Vensim PLE
Replies: 10
Views: 46

Re: Using data from the previous time // Vensim PLE

Looking at this equation, I see several other problems. For one thing, the constant .034 is embedded with no obvious interpretation, as is 9758. Unless the variables are dimensionless, this won't pass a units check. https://metasd.com/2019/11/seven-deadly-sins-sd-structure/ I would suggest a more ph...
by tomfid
Wed May 13, 2020 11:39 pm
Forum: Vensim
Topic: Random Function
Replies: 21
Views: 174

Re: Random Function

I think you need something more like this.
hazard ex 2.mdl
(3 KiB) Downloaded 4 times
by tomfid
Wed May 13, 2020 5:05 pm
Forum: Vensim
Topic: Random Function
Replies: 21
Views: 174

Re: Random Function

I'm still not clear on what the time dimension in your model represents - whether it is literal time or a proxy dimension for integrating over pdfs. Also, it's not clear why the HAZARDS and CF variables are stocks. If they are pdfs or hazard rates, they should be auxiliaries. Does the model represen...
by tomfid
Wed May 13, 2020 4:23 pm
Forum: Vensim
Topic: Delay conveyor sensitive to time step?
Replies: 3
Views: 56

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: 56

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: 174

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: 174

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: 174

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: 174

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: 174

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: 174

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: 174

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: 174

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: 174

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: 131

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)....