Search found 3800 matches

by tomfid
Fri Aug 25, 2023 2:24 pm
Forum: Vensim
Topic: Sensitivity analysis using the file option in a subscripted model
Replies: 13
Views: 705

Re: Sensitivity analysis using the file option in a subscripted model

What error are you seeing?

I'm guessing that you're putting varname[range] in the headers. I don't think that works - you need to provide a separate column for each varname[element].
by tomfid
Thu Aug 17, 2023 5:46 pm
Forum: Ventity
Topic: Entity activation time
Replies: 5
Views: 1820

Re: Entity activation time

Just walked through with the debugger.

The first try fails due to loading sequence and issues the error message too soon. The automatic retry works. The bug is that the error message is wrong -- there is not a real problem.
We can get you a nightly build when the fix is included.
by tomfid
Thu Aug 17, 2023 4:12 pm
Forum: Vensim
Topic: unit consistency bug
Replies: 5
Views: 1894

Re: unit consistency bug

Actually, the problem with "miles per trip" is that the equation is constant, but the diagram includes a link from "attractiveness of driving" so the equation is invalid (inconsistent with diagram). We should probably upgrade the warnings to distinguish missing equations from mis...
by tomfid
Thu Aug 17, 2023 4:05 pm
Forum: Vensim
Topic: unit consistency bug
Replies: 5
Views: 1894

Re: unit consistency bug

The others are either correct warnings, or Vensim cannot check as there is no equation to check against. Not so. "miles per trip" is a constant =20 and has unit mile/trip, but it's still reported as an error (no units). I suspect that the parsed model is so incomplete that Vensim fails to...
by tomfid
Wed Aug 16, 2023 1:56 pm
Forum: Ventity
Topic: any plans for a Mac version of Ventity?
Replies: 3
Views: 1956

Re: any plans for a Mac version of Ventity?

[ ] Almost done.
[ ] In the works.
[ X ] We're thinking about it
[ ] It's complicated.
[ ] No way, don't hold your breath

"Don't hold your breath" might still be good advice.
by tomfid
Wed Aug 16, 2023 1:55 pm
Forum: Ventity
Topic: any plans for a Mac version of Ventity?
Replies: 3
Views: 1956

Re: any plans for a Mac version of Ventity?

[ ] Almost done.
[ ] In the works.
[ X ] We're thinking about it
[ ] It's complicated.
[ ] No way, don't hold your breath

"Don't hold your breath" might still be good advice.
by tomfid
Wed Aug 16, 2023 12:18 am
Forum: Ventity
Topic: Entity activation time
Replies: 5
Views: 1820

Re: Entity activation time

The HG B/D mixup looks like a bug to me. Checking with the devs.

However, it's not clear to me that compound keys are needed here, at least for S1. Every S1 Factory is unique, so the group isn't needed to find the factory.
by tomfid
Wed Aug 16, 2023 12:08 am
Forum: Ventity
Topic: Entity activation time
Replies: 5
Views: 1820

Re: Entity activation time

I'll take a look.
by tomfid
Mon Aug 14, 2023 5:11 pm
Forum: Vensim
Topic: Help with a sigmoidal behaviour in a stock variable
Replies: 3
Views: 554

Re: Help with a sigmoidal behaviour in a stock variable

Sounds like what you want is a logistic growth model. The model here https://vensim.com/vensim-video-library/#Introduction_to_System_Dynamics_With_Vensim has a variant of that near the 1hr mark, particularly if you make the product lifetime very long so that there's effectively no outflow from custo...
by tomfid
Tue Aug 08, 2023 9:57 pm
Forum: Vensim
Topic: Units Check whack-a-mole
Replies: 9
Views: 1098

Re: Units Check whack-a-mole

Ventity licensing will be changing to look more like Vensim. In the meantime, we'll include a license with a Pub Rsch Vensim Pro license (email us at info at ventity.biz if it doesn't happen automatically).
by tomfid
Mon Jul 31, 2023 6:14 pm
Forum: Vensim
Topic: how do I reload an existing dataset, including variable values?
Replies: 5
Views: 540

Re: how do I reload an existing dataset, including variable values?

If you change variable values interactively, e.g. by moving sliders, the model always retains the underlying equation values as a default. If you change the equations, you have to manually change back. More advanced versions of Vensim can save and load change files (.cin) during simulations, to make...
by tomfid
Mon Jul 31, 2023 6:07 pm
Forum: Vensim
Topic: Units Check whack-a-mole
Replies: 9
Views: 1098

Re: Units Check whack-a-mole

After a closer look, my conclusion is that the model is kind of bonkers. Normally units prevent conceptual errors like mixing up stocks and flows, but in this case I think the general difficulty in defining units is a big red flag in itself. It means that the model is rather far from an operational ...
by tomfid
Mon Jul 31, 2023 4:36 pm
Forum: Vensim
Topic: Model calibration in Vensim PLE
Replies: 2
Views: 506

Re: Model calibration in Vensim PLE

No - PLE is really targeted at learning about dynamics.
by tomfid
Sun Jul 30, 2023 7:25 pm
Forum: Vensim
Topic: how do I reload an existing dataset, including variable values?
Replies: 5
Views: 540

Re: how do I reload an existing dataset, including variable values?

Rather than editing equations, you probably want to make your changes to the model live, in Synthesim, or using change files.
by tomfid
Sun Jul 30, 2023 1:10 am
Forum: Vensim
Topic: Units Check whack-a-mole
Replies: 9
Views: 1098

Re: Units Check whack-a-mole

I'll take a look in greater depth Monday, but I tend to agree that these errors reflect some conceptual issues with the model. I find the rewilding equation extremely weird. I'd probably fix the units by adding an explicit parameter, with value 1, to make clear that there's an implicit conversion go...
by tomfid
Sun Jul 30, 2023 12:57 am
Forum: Vensim
Topic: Reverse engineering
Replies: 3
Views: 491

Re: Reverse engineering

Generally a causal loop diagram can't be translated automatically to a model, because it simply doesn't contain enough information. The COIN diagram you posted actually has an underlying model that is much larger. https://metasd.com/2010/04/hypnotizing-chickens/ You should be able to turn a CLD into...
by tomfid
Fri Jul 28, 2023 9:46 pm
Forum: Vensim
Topic: Units Check whack-a-mole
Replies: 9
Views: 1098

Re: Units Check whack-a-mole

A stock equation is the integral of a flow over time, so the stock unit has the flow unit * time unit. This is easier to see if you write the discrete update that happens with Euler integration: stock(t+dt) = stock(t) + dt*(inflows-outflows) were t=time, dt=TIME STEP in Vensim. This has units: stock...
by tomfid
Fri Jul 28, 2023 9:19 pm
Forum: Vensim
Topic: Input Mode
Replies: 2
Views: 440

Re: Input Mode

I'm not sure what you mean by "input mode".

The conventional way to implement exponential growth is:

stock = INTEG( - outflow, initial stock ) ~ stuff
outflow = stock * loss rate ~ stuff/time

or

outflow = stock / lifetime
by tomfid
Fri Jul 28, 2023 9:19 pm
Forum: Vensim
Topic: Input Mode
Replies: 2
Views: 440

Re: Input Mode

I'm not sure what you mean by "input mode".

The conventional way to implement exponential growth is:

stock = INTEG( - outflow, initial stock ) ~ stuff
outflow = stock * loss rate ~ stuff/time

or

outflow = stock / lifetime
by tomfid
Fri Jul 28, 2023 2:38 am
Forum: Vensim
Topic: Units Check whack-a-mole
Replies: 9
Views: 1098

Re: Units Check whack-a-mole

The Log(Pop) error can be handled by defining something like unit pop == 1 ~ person and then normalizing logpop = log( pop/unit pop + 1 ) The other two errors seem like a unit problem with knowledge. Flows always have units of their associated stock, divided by the time unit. Therefore if learning a...
by tomfid
Thu Jul 27, 2023 5:01 am
Forum: Vensim
Topic: Exogenous data in transportation model
Replies: 8
Views: 1346

Re: Exogenous data in transportation model

PLE doesn't include the stats tool unfortunately. You could generate a table and copy/paste to Excel or something for the comparison.
by tomfid
Thu Jul 27, 2023 4:58 am
Forum: Vensim
Topic: The use of 'TIME STEP' in equations
Replies: 8
Views: 670

Re: The use of 'TIME STEP' in equations

However ... the TREND function does essentially what I proposed, without requiring the extra equation.
by tomfid
Tue Jul 25, 2023 9:43 pm
Forum: Vensim
Topic: The use of 'TIME STEP' in equations
Replies: 8
Views: 670

Re: The use of 'TIME STEP' in equations

It sounds like you're trying to recover the value of a variable at a previous time step? This appears to be computing a growth rate or derivative? You could write something like: Previous Resident Income = SMOOTH(Resident Income,TIME STEP) income trend = ( (Resident Income - Previous Resident Income...
by tomfid
Mon Jul 24, 2023 1:22 am
Forum: Vensim
Topic: Response Time
Replies: 0
Views: 5500

Response Time

Many of us are at #ISDC2023 so response times will be longer this week.