Ventana software support forum

You can run your own algorithm via the .dll, but there's quite a bit of overhead involved in setting it up, and you'll lose at least a factor of 2 in speed per iteration. The Powell optimizer initially searches directions one at a time, but the search is iterative, so that sequential updating of a,b...

This is inherent to continuous time simulation. There's an example of an alternative with the SHIFT IF TRUE function in the help system. http://www.vensim.com/documentation/fn_shift_if_true.htm See also Eberlein, Robert with James Thompson Chronological Aging in Continuous Time http://www.systemdyna...

If you have a stock of installed items, and an outflow of failures proportional to the stock, the flow is the expected value of an exponential distribution. Might look like this: installed base = INTEG(selling - failing, init installed base) selling = ... failing = installed base/mean lifetime You c...

It really depends on what question about the supply chain you're trying to answer. If it's something very detailed, like how many of a given type of machine to use, a discrete event simulation might be in order. But if it's strategic, like how to combine pricing and marketing with production schedul...

It might help us help you if you state what you're trying to accomplish here.

1. TIME STEP should definitely be as short or shorter than the sampling interval of your data. Stability is easy to verify empirically - just cut your time step in half and see if the behavior changes (it shouldn't, unless there are stochastic elements in your model). A general rule of thumb is that...

Here's a refinement. It's not simpler, but conceptually cleaner in a few respects. - parameters embedded in equations converted to explicit variables with units (tank capacity) - variable names indicate a quantity that can vary (water in tank) - water use limited to water available (a term could be ...

Optimization: http://www.vensim.com/documentation/usr18.htm The purpose of the optimizer is for calibrating dynamic models and optimizing policies. You can use it to fit simple curves, but it's not the most efficient tool for that job. A lookup won't help you to fit a distribution, though you could ...

They are hidden by default because they don't affect the dynamic structure of the model. I actually prefer things the other way, so I generally use the Model>Settings to turn them on, then manually hide any arrows that cause visual clutter. Then you can show/hide them as needed by hitting the H key ...

Check the Help system for the Random functions - there are examples for RANDOM UNIFORM and RANDOM NORMAL that should do what you need.

You can use the optimizer for curve fitting, but for simple, non-dynamic tasks like fitting a distribution, you're probably better off using some other tool.

Another possibility is to create reporting variables that suppress a portion of a variable's history. For example:

reported x = IF THEN ELSE( Time >= threshold, x, :NA: )

PLE allows import of data for comparison, but not data variables for driving the model. You can put your data in a lookup instead.

Almost all built-in functions have a sample model linked to the function's page in the Help system.

I think it's important, but the quality of the model is of greater importance. I can make sensitivity runs with your model without any problems on 6.0b on a Windows 7 machine, so I'm not sure what the problem is. Also, I don't get any warnings about integration errors, because the integration method...

Here are two Vensim possibilities:
http://vensim.com/building-a-simple-vensim-model/
http://www.vensim.com/documentation/users_guide.htm

And the System Dynamics bible:
http://amzn.com/007238915X

I still can't replicate any problems - this works fine for me. What Vensim version and operating system are you running?

Hmm ... this runs fine for me. Can you post one more file - the savelist (.lst)? Thanks.

Also, we'll need any input data that the model requires to run - either a .vdf or .xls, per usage.

I think we can't debug the DCC43327 error without a copy of the model and sensitivity files - you can email them to us, vensim at vensim.com if you can't post them publicly.

Set your warnings to "Display" to debug the data first time and integration errors.

The "before first time" error just means that your model starts before the first data point for some data variable. Check to be sure that you have Tools>Options>Settings>Warnings set to "Display." I'll look into the other.

The first errors are warnings from RK2 or RK4 auto integration. They may not be important. Do you get any of these warnings in an ordinary simulation? If not, you may have to try some parameter variations matching your sensitivity file in order to recreate the problem. The "before first time......

Example:

Yes - you could do this with a combination of SAMPLE IF TRUE and VECTOR LOOKUP.

I'm also working on an external function library that might help with this.

Tom

It would be interesting to see your vdi and vdo too - especially the first.