Ventana software support forum

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...

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.

Vensim's text capabilities are pretty limited. What are you trying to accomplish?

http://models.metasd.com/world3-population-sector/
http://www.systemdynamics.org/conferenc ... /P1064.pdf

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...

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.

I would avoid the xfn approach if possible, given the platform constraints.

A simplified example would help - even a broken one.

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 ...

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.

This is also relevant to economic models:
http://blog.metasd.com/2014/06/early-ec ... celerator/

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)

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...

Yes.

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.

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.

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.

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...

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...

We could probably give better advice if you explained exactly what situation you're trying to model here.

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...

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...

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

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!

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.