Frequently Asked Questions

If you don't get the answer you want here check out the discussion forum at http://www.ventanasystems.co.uk/forum.

Questions about Vensim

Conceptual Questions

Conceptual Questions about feedback, systems thinking, and system dynamics are on another page.

If you have any further questions, please email us at faq@vensim.com

What are the limitations on Vensim PLE?

Vensim PLE has no built-in limitations.  It it a simplified version of Vensim designed for learning how to build and analyze models.  We removed all functionality not relevant to learning modeling skills.  Very few settings or options can be changed making the software behave the same for everyone.  Vensim PLE is a serious tool for building and using system dynamics models.  For many people it will be completely sufficient. Back to top

Why is Vensim PLE free?

Vensim PLE is free for academic use because we want to make it easier for more people to learn system dynamics.  We especially want to lower the barriers of entry for students.  We also hope that people who learn with Vensim PLE will start to use the modeling approach seriously in their work and therefore pay the shareware license fee, or choose to purchase a more advanced version of Vensim.  Back to top

What are the differences among PLE, PLE Plus, Professional and DSS?

All the products share essentially the same interface and each is a superset of the one that precedes it.  Vensim PLE Plus adds to PLE easier use of data, and Monte Carlo or sensitivity simulation capabilities.  Professional allows you to use subscripts to represent more detail with less effort, and also supports optimization.  Vensim DSS include the ability to construct management flight simulators and also to use external functions and compiled simulations.  Back to top

What are the hardware requirements?

The Vensim family of software runs on Windows and the Macintosh and is compatible with Windows 95/98/NT/2000/XP and the Power Macintosh running System 7 or higher. Vensim requires about 20 megabytes of disk space for a full installation and will run with any reasonable amount of memory.  For very large models more memory and disk space will be required as a single run can easily take 100 megabytes of disk space.  Generally it is a good idea to have at least as much memory as the size of the largest .vdf file you are creating. 

How soon can I get the software?

We normally ship products within one day of receiving the order.  In the US we ship via priority mail and it normally takes 2 days.  For international orders, shipment times vary.  More information on purchasing.  Back to top

How does Vensim compare to other system dynamics software?

For model building, Vensim and other system dynamics languages have a great deal in common.  The available functions and default graphical presentations are similar.  Vensim is more flexible than most in the appearance of the model diagram, allowing you to easily mix stock and flow and causal loop elements.

On the analysis side, Vensim is unique.  It contains a set of Analysis tools that use the structure of the model to present information to quickly find problems and investigate sources of behavior.  Our patented Causal Tracing^® capabilities allow you to instantly see the behavior of a variable and the variables that connect to it.

Vensim is also very strong in terms of capacity, performance and functionality.  Simulation speed is fast, the optimization capabilities are powerful.  The sensitivity analysis is both fast and powerful, there are no practical limits on model size, and it is easy to extend the base capabilities using external functions of the Vensim DLL.  Back to top

What is an External Function?

Vensim has a large number of built in functions, but these are not always enough.  Suppose, for example, you would like to allocate production among different facilities at each time in a simulation.  If the best way to do this is to use a Linear Program, then an external function can be constructed to do this.  The external function is written in a programming language (usually C or C++) and then compiled into a dynamic link library (DLL) which is loaded by Vensim.  Back to top

What is a Compiled Simulation?

Compiled simulation is a technique for speeding up simulation.  The model equations are written out as a C program which is then compiled and linked with Vensim as a DLL.  Fast simulation is especially important for optimization.  Back to top

How does Subscripting work?

Subscripting allows you to create a piece of model structure and equations (for example, a store) then "subscript" it to produce multiple stores, all with the same structure but with their own constant values, inputs, and outputs.  Read more about subscripting  Back to top

How does Optimization work?

Optimization can be used in several ways in Vensim.  Model calibration allows you  to take external "real world" data and then optimize model parameters (constants) so that the model best fits the external data.  Policy optimization allows you to maximize (or minimize) one or many policy parameters.  For example, you might optimize multiple parameters such as worker numbers, capital spending, etc., to maximize profit while minimizing production time, and all constrained by certain limits.  Optimization is performed using a modified Powell hill climbing algorithm.  Read more about optimization  Back to top

Will you include a genetic algorithm based optimization?

We have experimented extensively with genetic algorithm optimization and found that the results are very poor.  We will be making it easier to add in alternative search algorithms in the future for people who want to pursue this line of research.  Back to top

How can I get a matrix of values into Vensim from a spreadsheet?

How can Vensim communicate with other programs?

There are a number of options for communication, starting with the clipboard.  Vensim can also easily import or export data and constants from other sources.  For dynamic control of Vensim's behavior, the Vensim DLL allows you to control Vensim from Visual Basic, Delphi or any other programming language.  Back to top

How can I get Vensim graphics into other applications?

In almost all cases just use Edit>Copy in Vensim (or Ctrl+C) and then paste them into your other application.  For tool output Vensim also has a little button you can click on to do this.  Back to top

How big can a Vensim model be?

With the exception of Vensim for Windows 3.1, there are no limitations to the size of models.  Some people have models with excess of 1 million variables when all subscript dimensions are expanded.  Such models take a long time to simulate and create very large output datasets, and therefore need to be treated with care.  Back to top

What are Analysis tools?

Analysis tools act on a selected variable to show something: a graph of behavior, a tree diagram of connected variables, the variable's equation, all the feedback loops passing through it, and other information. Back to top

What is Causal Tracing®?

Several analysis tools allow you to perform Causal Tracing®  A Causes Tree diagram shows a selected variable and the variables that "cause" it to change (up to 3 connections distant — this is adjustable).  Selecting a variable in this tree and clicking the Causes Tree tool again creates a tree diagram of that variable.  In this way, you can trace the causes (or uses) of a variable throughout a model.  A Causes Strip Graph displays behavior for the selected variable and all the variables directly connected to it so that you can see which variables contribute which types of behavior.  Selecting one of these variables and clicking the Causes Strip Graph again traces the behavior for that variable and variables directly connected to it.  You can continue to trace sources of behavior throughout your model.  This is useful for discovering which feedback loops contribute certain types of behavior in a model (growth, decay, etc.).  Read more about Causal Tracing  Back to top

What is a Dataset?

Simulations produce a "dataset" of output values for every variable in the model.  This allows you to do "scientific experiments" on the model, saving each simulation as a different named experiment for later comparison.  No need to perform repeated simulations just to look at other variable's behaviors.  Graph showing multiple experiments  Back to top

How can I get variable names to appear on a single line?

By default, variables are entered in clear boxes.  For variables already created, right click (or control click) on the variable name and select the shape "None"  For variables you are putting onto the sketch using the Variable tool, change the option for the tool to Shape "None". Or go to Tools>Options>Sketch and uncheck "Start Words in Clear Box."  Back to top

How do I draw bent or split rate pipes?

Bent Pipes: Start the rate as usual (select the Rate tool and click once on a blank part of the sketch or on a level), then move the cursor to a place you want the pipe bent, hold down the shift key and click once on the sketch.  This drops a "corner".  If you want more bends/corners, keep holding down the shift key and click on the sketch.  Finish the rate by clicking without holding the shift key down.
Split Pipes: Rate pipes can start or stop at three places: at a cloud on the sketch, at a level, and at a valve.  If, for example, you want a single rate to come out of one level and then to split and feed into two levels, this is how you do it.  Create the three levels.  Join two together with a rate.  Again with the Rate tool, click on the valve of the first rate, then hold the shift key and click on a blank part of the sketch near the third level, then click on the third level.  (This makes Vensim put the rate pipe on the sketch and bend it near the third level.  If you don't do this, you might find that Vensim draws the pipe with its bend through the second level, because pipes can only be horizontal or vertical.)
You can also make the illusion of a pipe splitting off somewhere other than a valve (see page 22 of the Vensim Modeling Guide) by clicking on the valve, then creating a bend on top of the existing pipe, then making another bend near the level and finishing on the level.  The two pipes will be drawn exactly on top of each other, creating the illusion of a single pipe at that point.

How do I delete an arrow?

The packman icon deletes things — just click on the arrowhead with it (clicking on the handle no longer deletes the arrow).

How do I connect a box variable to the cloud end of a previously-inserted rate?

1) If you are wanting to join the arrowhead end of a rate (that drops into a cloud), using the Hand tool, drag the arrowhead of the rate onto your box variable, or 

2) If you don't have an arrowhead (pipe comes out of a cloud), or even if you do have an arrowhead, using the Merge tool, drag and drop the box variable onto the arrowhead or cloud.

3) PLE and PLE Plus do not have a Merge tool.  To connect a box variable to the cloud of a previously inserted rate, select the Rate tool, click on the box variable and then click on the valve of the rate (not on the cloud). Then use the Delete tool to delete the cloud.