Search found 3808 matches
- Tue Jul 08, 2014 4:34 pm
- Forum: Vensim
- Topic: MIT System Dynamics Group seeks new tenure track faculty mem
- Replies: 0
- Views: 4130
- Tue Jul 08, 2014 4:33 pm
- Forum: System Dynamics Discussion
- Topic: MIT System Dynamics Group seeks new tenure track faculty mem
- Replies: 0
- Views: 4809
MIT System Dynamics Group seeks new tenure track faculty mem
https://sloanfacultysearches.mit.edu/sd/ The MIT Sloan School of Management invites applications for a tenure-track faculty position in system dynamics, to begin July 2015 (or thereafter). Candidates should have excellent knowledge of the system dynamics simulation technique and/or related modeling...
- Tue Jul 08, 2014 4:02 am
- Forum: Vensim
- Topic: Contractor Opportunity [No Longer Accepting Applications]
- Replies: 2
- Views: 2128
Re: 6 Month+ Vensim Contractor Opportunity
Not at all. Seems appropriate. You might also post on the SD Forum and the LinkedIn Vensim Users Group. I'd be happy to copy this there if helpful.
- Tue Jul 08, 2014 4:00 am
- Forum: Vensim
- Topic: Using if then else with Strings
- Replies: 2
- Views: 2182
Re: Using if then else with Strings
Vensim's text capabilities are pretty limited. What are you trying to accomplish?
- Mon Jul 07, 2014 3:48 pm
- Forum: Vensim
- Topic: Need help with demography model
- Replies: 5
- Views: 2984
- Mon Jul 07, 2014 3:46 pm
- Forum: Vensim
- Topic: Need help with demography model
- Replies: 5
- Views: 2984
Re: Need help with demography model
http://www.vensim.com/documentation/mgu09_population_example_with_conveyors.htm?zoom_highlightsub=population http://www.vensim.com/documentation/fn_shift_if_true.htm?zoom_highlightsub=population http://www.vensim.com/documentation/vendemo_build.htm?zoom_highlightsub=population http://www.vensim.com...
- Mon Jul 07, 2014 1:06 pm
- Forum: Vensim
- Topic: Need help with demography model
- Replies: 5
- Views: 2984
Re: Need help with demography model
Look in the help system - there are several population model examples in the User Guide and reference material, including the world3 population sector.
Also, check the System Dynamics Conference proceedings online - there are probably many examples.
Also, check the System Dynamics Conference proceedings online - there are probably many examples.
- Wed Jul 02, 2014 10:28 pm
- Forum: Vensim
- Topic: Difficulty Handling Matrix Operation
- Replies: 7
- Views: 4557
Re: Difficulty Handling Matrix Operation
I would avoid the xfn approach if possible, given the platform constraints.
A simplified example would help - even a broken one.
A simplified example would help - even a broken one.
- Wed Jul 02, 2014 6:44 pm
- Forum: Vensim
- Topic: Difficulty Handling Matrix Operation
- Replies: 7
- Views: 4557
Re: Difficulty Handling Matrix Operation
I think you can simplify this problem by dispensing with the [source x source] matrix. Plants coming online or going offline don't really have any privileged relationship to one another; they just add and subtract total capacity. Then, generically, you could have logic that (a) brings on economical ...
- Fri Jun 27, 2014 8:22 pm
- Forum: Vensim
- Topic: Uploading Data onto Vensim PLE
- Replies: 27
- Views: 32785
Re: Uploading Data onto Vensim PLE
You should get the same answer either way. The values in the first two time steps won't be established by the data, but as long as you initialize the lag states to a reasonable value, they should be fine. Or you could just start the model at time 2.
- Fri Jun 27, 2014 12:32 am
- Forum: Vensim
- Topic: Error Correction Model
- Replies: 3
- Views: 2212
Re: Error Correction Model
This is also relevant to economic models:
http://blog.metasd.com/2014/06/early-ec ... celerator/
http://blog.metasd.com/2014/06/early-ec ... celerator/
- Thu Jun 26, 2014 3:45 pm
- Forum: Vensim
- Topic: Uploading Data onto Vensim PLE
- Replies: 27
- Views: 32785
Re: Uploading Data onto Vensim PLE
Where the lag is one period (i.e. one TIME STEP) and you're using Euler or Diff integration, you can also write:
Lagged C = SMOOTH(C,one year)
or
Lagged C = INTEG((C-Lagged C)/one year,one year)
Lagged C = SMOOTH(C,one year)
or
Lagged C = INTEG((C-Lagged C)/one year,one year)
- Thu Jun 26, 2014 3:25 pm
- Forum: Vensim
- Topic: Uploading Data onto Vensim PLE
- Replies: 27
- Views: 32785
Re: Uploading Data onto Vensim PLE
If you want to use econometric notation directly, you're probably better off to use R. But you can implement lags directly in Vensim. Generically, if you have something like deltaC(t) = C(t)-C(t-1) = .5*Y(t) - .2*(C(t-1)-.9*Y(t-1)) + expsilon (the ECM at http://en.wikipedia.org/wiki/Error_correction...
- Thu Jun 26, 2014 2:49 pm
- Forum: Vensim
- Topic: Uploading Data onto Vensim PLE
- Replies: 27
- Views: 32785
- Thu Jun 26, 2014 2:48 pm
- Forum: Vensim
- Topic: Error Correction Model
- Replies: 3
- Views: 2212
Re: Error Correction Model
Yes - though again, the continuous time, behavioral approach in system dynamics is generally far superior to the typical discrete time econometric notation for the same problem.
- Wed Jun 25, 2014 5:51 pm
- Forum: Vensim
- Topic: Uploading Data onto Vensim PLE
- Replies: 27
- Views: 32785
Re: Uploading Data onto Vensim PLE
This looks much better. To follow SD conventions, I'd use something other than a box to identify the 4 "total" variables, since they are (correctly) no longer stocks (INTEG). I obviously can't speak to the numerical details, but this seems like a reasonable structure.
- Tue Jun 24, 2014 9:56 pm
- Forum: Vensim
- Topic: Vensim DSS compilation error
- Replies: 9
- Views: 6013
Re: Vensim DSS compilation error
A humble suggestion: 5.7 is very old. There have been three operating system versions, six major Vensim versions, and several Visual Studio versions since then. If you need speed, and you're going to invest the effort to update the external functions, a maintenance update might be a good idea first.
- Tue Jun 24, 2014 4:53 pm
- Forum: Vensim
- Topic: Uploading Data onto Vensim PLE
- Replies: 27
- Views: 32785
Re: Uploading Data onto Vensim PLE
To model a lag, you can use DELAY FIXED in advanced version. If you're using Euler or Diff integration with a time step of 1, you can also use lagged x = INTEG( (current x - lagged x)/TIME STEP, ... ) or lagged x = SMOOTH( current x, TIME STEP) To be blunt, the typical notation for vector autoregres...
- Mon Jun 23, 2014 3:33 pm
- Forum: Vensim
- Topic: Uploading Data onto Vensim PLE
- Replies: 27
- Views: 32785
Re: Uploading Data onto Vensim PLE
The basic problem here is exactly what the error message suggests - that lnrgdp f is used as a lookup, but its equation suggests that it is an auxiliary depending on rate of flow. I think this model is going to have problems, even if it runs. There are two integrations between availStk and GDP -> pr...
- Wed Jun 18, 2014 4:10 pm
- Forum: Vensim
- Topic: Input data for sensitivity analysis
- Replies: 15
- Views: 7799
Re: Input data for sensitivity analysis
We could probably give better advice if you explained exactly what situation you're trying to model here.
- Wed Jun 18, 2014 1:44 am
- Forum: Vensim
- Topic: Input data for sensitivity analysis
- Replies: 15
- Views: 7799
Re: Input data for sensitivity analysis
If only the last point is being used, that sounds more like a problem with time alignment of your data than with the sensitivity tool, but I've never tried using data variables as sensitivity ranges. In any case, you can't expect the sensitivity draw to vary dynamically over time; it only sets up co...
- Wed Jun 18, 2014 1:38 am
- Forum: Vensim
- Topic: subscript within a subscript
- Replies: 1
- Views: 1682
Re: subscript within a subscript
Sure. The .mdl format equations would be like: store : shoestore, hatstore ~~| shoestore : shoes, cowboy boots, laces, insoles, outsoles ~~| hatstore : ballcaps, cowboy hats ~~| You could also add a subrange that overlaps categories, like: ranch supply : cowboy boots, cowboy hats ~~| You can go to m...
- Thu Jun 12, 2014 2:13 pm
- Forum: Vensim
- Topic: Shift if true.mdl
- Replies: 8
- Views: 4223
Re: Shift if true.mdl
Right ... it's (775,000-433,000) people/year for 1/8 year in the first time step, so the expected change is 43,000 - which is what you get.
With Euler integration:
stock(t+dt) = stock(t) + (inflow-outflow)*dt
with units
people = people + (people/year)*year
where dt = time step = .0125
With Euler integration:
stock(t+dt) = stock(t) + (inflow-outflow)*dt
with units
people = people + (people/year)*year
where dt = time step = .0125
- Wed Jun 11, 2014 9:43 pm
- Forum: Vensim
- Topic: Shift if true.mdl
- Replies: 8
- Views: 4223
Re: Shift if true.mdl
There may be a bug in SHIFT IF TRUE in your version - I get the expected decrease of about 500/yr when I run with 6.3. Time for a maintenance extension - 5.0 is ten years old!
- Wed Jun 11, 2014 9:31 pm
- Forum: Vensim
- Topic: Shift if true.mdl
- Replies: 8
- Views: 4223
Re: Shift if true.mdl
Here's a model with a checksum that is what I'd expect from your description. There are slight numerical discrepancies - about .01% - presumably due to roundoff. Nothing like what you're describing. As an extra check, the final population should be 172.5M when you run this.