Search found 2182 matches

by tomfid
Thu Apr 19, 2018 7:51 pm
Forum: Vensim
Topic: Potential doublecounting in a example model
Replies: 8
Views: 64

Re: Potential doublecounting in a example model

I agree that there is also a delay between existence of a gap (desired-actual houses), but I think in that case a smooth is not the best way to represent it. It would be better to represent price formation explicitly. However, if that's not the focus of the model, a smooth, or nothing, might be OK.
by tomfid
Thu Apr 19, 2018 6:22 pm
Forum: Vensim
Topic: How to set "bidirectional" flow for a stock
Replies: 3
Views: 18

Re: How to set "bidirectional" flow for a stock

You can turn on the second arrow by right-clicking the pipe, but I don't find it helpful.
by tomfid
Thu Apr 19, 2018 6:20 pm
Forum: Vensim
Topic: Potential doublecounting in a example model
Replies: 8
Views: 64

Re: Potential doublecounting in a example model

In a typical housing market, the term for replacement of demolition probably has low weight. In other words, the market doesn't notice the outflow directly; it only responds when the gap has grown big enough to affect prices. That, plus the inability to perceive the construction pipeline easily, mak...
by tomfid
Thu Apr 19, 2018 5:31 pm
Forum: Vensim
Topic: How to set "bidirectional" flow for a stock
Replies: 3
Views: 18

Re: How to set "bidirectional" flow for a stock

Vensim flows are always bidirectional, unless you explicitly constrain the equation with a MAX(0,...) or similar. For example, expected price = INTEG( expectation adj rate, initial price ) ~ $/kg expectation adj rate = ( actual price - expected price )/adj time ~ $/kg/week On the diagram you'd show ...
by tomfid
Thu Apr 19, 2018 5:26 pm
Forum: Vensim
Topic: Potential doublecounting in a example model
Replies: 8
Views: 64

Re: Potential doublecounting in a example model

Good question. Substituting the equations into one, you have: Planning Rate = MAX( 0, demolishing + (number of houses required - Houses Completed) / time to respond to gap ) If you start with number of houses required = Houses Completed, the second term will be 0. Therefore the planning rate will ju...
by tomfid
Thu Apr 19, 2018 2:22 pm
Forum: Vensim
Topic: quastion about Limiting Domain
Replies: 2
Views: 18

Re: quastion about Limiting Domain

The min and max setting provides warnings but doesn't change the dynamics.

There is a SINTEG function that has min/max limits, but I wouldn't recommend using it, because it potentially violates conservation laws.
by tomfid
Thu Apr 19, 2018 2:20 pm
Forum: Ventity
Topic: Lookup or time series for data input
Replies: 4
Views: 73

Re: Lookup or time series for data input

We're exploring options on this. After some reflection, I'm wondering why you need to run hourly for 30 years. I'm assuming this is electric power, but no matter what the domain, running a tactical time step on a strategic horizon is tough. It implies that the long term model has to have a lot of sh...
by tomfid
Wed Apr 18, 2018 1:57 pm
Forum: Ventity
Topic: Lookup or time series for data input
Replies: 4
Views: 73

Re: Lookup or time series for data input

If you have a file that's big enough to choke the Ventity data input, I'd love to have a copy for testing.
by tomfid
Tue Apr 17, 2018 11:35 pm
Forum: Vensim
Topic: Vensim constant behavior
Replies: 7
Views: 45

Re: Vensim constant behavior

The model will not generally be stable unless the time step is shorter than the shortest delay. Powers of 2 are generally better, so perhaps .03125 would be a good start (you can set this via the dropdown menu at Model>Settings>Time Bounds.
by tomfid
Tue Apr 17, 2018 3:16 pm
Forum: Ventity
Topic: Lookup or time series for data input
Replies: 4
Views: 73

Re: Lookup or time series for data input

Understood. I've put in a ticket for an improvement that addresses this.

In the meantime, what if you create a time series that repeats the lookup data N times?
by tomfid
Sun Apr 15, 2018 4:24 pm
Forum: Vensim
Topic: sensitivity analyse
Replies: 2
Views: 32

Re: sensitivity analyse

PLE doesn't, because it's targeted narrowly at learning about dynamics. PLE+ does.
by tomfid
Fri Apr 13, 2018 3:21 pm
Forum: Vensim
Topic: ALLOCATE BY PRIORITY - problems with managing allocations
Replies: 21
Views: 255

Re: ALLOCATE BY PRIORITY - problems with managing allocations

There are sample models for all the ALLOC functions in the Help system.
by tomfid
Wed Apr 11, 2018 6:23 pm
Forum: Vensim
Topic: lookup table
Replies: 16
Views: 112

Re: lookup table

Indeed you are correct. A growth rate dependent on time might be convenient if you just want a GDP scenario as an input to a model of something else, but it isn't much if you're interested in modeling the economy itself.
by tomfid
Wed Apr 11, 2018 3:59 pm
Forum: Vensim
Topic: lookup table
Replies: 16
Views: 112

Re: lookup table

JJ - I think it's a terminology issue. In my suggested equations, "growth rate" refers to the fractional rate of increase, in units of fraction/year, as distinct from the actual growth that occurs, in $/year/year. So, the full equations might be: GDP = INTEG( GDP Growth, Initial GDP ) ~ $/...
by tomfid
Tue Apr 10, 2018 9:57 pm
Forum: Vensim
Topic: lookup table
Replies: 16
Views: 112

Re: lookup table

To suppress units warnings you can make the x-input Time/Unit Time (where Unit time = 1 with whatever time unit you need). Then:

GDP Growth Rate = GDP Growth Lookup( Time/Unit Year ) ~ fraction/year
GDP Growth Lookup ... {table}
Unit Year = 1 ~ year
by tomfid
Tue Apr 10, 2018 9:53 pm
Forum: Vensim
Topic: Venapp
Replies: 2
Views: 30

Re: Venapp

Another option worth looking at is Sable, which gives a cleaner-looking result than VenApp.
by tomfid
Fri Apr 06, 2018 2:38 pm
Forum: Vensim
Topic: Where is "vendll32.dll" file?
Replies: 10
Views: 462

Re: Where is "vendll32.dll" file?

Also, be aware that this kind of use probably requires a separate license for the server machine - a regular DSS license if it's doing batch work internal to your organization, and a Multicontext server DLL license if it's running models directly for the outside world (www). In some cases a redistri...
by tomfid
Fri Apr 06, 2018 3:40 am
Forum: Vensim
Topic: Can I take in input data paths from another input data file?
Replies: 12
Views: 419

Re: Can I take in input data paths from another input data file?

I think you've identified a capability gap, so I'll put this on the todo list.
by tomfid
Thu Apr 05, 2018 6:12 pm
Forum: Vensim
Topic: error in simulation data
Replies: 7
Views: 284

Re: error in simulation data

You have ((Internet users-number of internet site visitor)/Internet users) for both the inflow and outflow of that stock, so the net change is 0. Having no units is not really an improvement over having units errors. I'd suggest working through some of the examples in the User Guide section of the H...
by tomfid
Thu Apr 05, 2018 4:06 pm
Forum: Vensim
Topic: GET DATA LAST TIME... with auxiliary
Replies: 1
Views: 68

Re: GET DATA LAST TIME... with auxiliary

From your description, it sounds like "BIM investment cost" must be somehow dependent on whether the investment is taken or not ("BIM ok"), which might be the problem. However, it's hard to say without a more concrete example. As a general principle, using information from the fu...
by tomfid
Wed Apr 04, 2018 6:01 pm
Forum: Vensim
Topic: Where is "vendll32.dll" file?
Replies: 10
Views: 462

Re: Where is "vendll32.dll" file?

You do need the .dll. It's probably in "C:\Windows\SysWOW64" which should be in Java's search path. If it's not, you can set the Java path environment variable to point there.
by tomfid
Wed Apr 04, 2018 2:10 pm
Forum: Vensim
Topic: Importing data into PLE
Replies: 4
Views: 121

Re: Importing data into PLE

There are a couple exceptions: - You can use Model>From Dat Format to import data in the .dat format (see the Help system), but you can only use that for comparisons; you can't create a data variable for driving calculations - The Reference Mode tool permits you to draw data - You can put data in a ...
by tomfid
Tue Apr 03, 2018 3:29 pm
Forum: Vensim
Topic: error in simulation data
Replies: 7
Views: 284

Re: error in simulation data

Also, remove/replace the dynamic connection from "penetration rate" to "internet users" - stocks can only change via flows.
by tomfid
Tue Apr 03, 2018 3:26 pm
Forum: Vensim
Topic: GET XLS SUBSCRIPT : vdf conversion ?
Replies: 13
Views: 422

Re: GET XLS SUBSCRIPT : vdf conversion ?

That could be a 32 vs 64 bit issue. There's supposed to be a 32-bit connector process for 64-bit Excel, but it may not be working.

If you use GET DIRECT instead of GET XLS, you don't have to have Excel in the loop.

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