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. There is a comparison chart here. Back to top
What are the hardware requirements?
The Vensim family of software runs on Windows
98/NT/2000/XP/Vista/7. Vensim is a 32 bit progam but will run
fine on 64 bit versions of Windows. The Macintosh version requires OSX
version 10.4 or higher on an Intel processor. The Windows Version
will run on the Macintosh using emulators like VMware or Crossover, or
with a Windows virtual/alternate boot installation. 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, online order are processed in real time. If
you select electronic delivery an email is sent with download and
install instructions and you can start using the product right away. In
the US we ship CDs 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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