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?
Vensim (except PLE) supports the importation of times series
data from spreadsheets, but if you are just looking at one time (either
for initial conditions or constants) this is not efficient. To set
for, for example, initial population from a spreadsheet, set the spreadsheet
up in the form:
USA MEX CAN
initial population[country,male] 1 2 3
initial population[country,female] 2 3 4
Save the spreadsheet as a CSV (comma delimited) file. The first row,
because it contains no values in the first column, is treated as a comment.
Then in the model just enter an equation such as:
initial population[country,sex]=1 ~Person~|
Then enter the CSV file you created from the spreadsheet as a changes file
in the simulation control dialog. Note that it is important to be
sure that the order of subscripts in the subscript definition
country: USA,MEX,CAN ~~|
is the same as the order in the spreadsheet. Back
to top
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.
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