Category Cross-Omics>Agent-Based Modeling/Simulation/Tools

Abstract Vensim® simulation software is used for developing, analyzing, and packaging high quality dynamic feedback models.

Models are constructed graphically or in a text editor. Features include dynamic functions, subscripting (arrays), Monte Carlo sensitivity analysis, optimization, data handling, application interfaces, and much more.

Ventana Systems also provides the Molecules software (see below...), for constructing system dynamics models from "chunks" or molecules of system dynamics structure.

The Power of Vensim -- Nothing is easier than Vensim for creating customized causal loop or stock and flow diagrams. Nothing is more efficient than Vensim for building accurate simulation models of dynamic feedback systems.

Building Models -- With Vensim, you can customize diagrams with different colors, fonts, symbols, arrows, and pipes. Variable names can appear alone, or inside or outside of boxes, circles, hexagons, and other shapes. You can create multiple views in one model with each view containing a portion of the total model structure.

An Equation Editor helps you build the equations for a simulation model. Vensim can create and simulate models with hundreds of thousands of variables.

Vensim has many built-in functions including user defined Lookups, test input patterns, logical operators, random number generators, continuous and discrete delays, smooths and forecasts, 'scientific functions', and customizable Vensim macros and external functions.

Simulation -- Vensim contains a highly efficient simulation engine providing fast simulation times and allowing storage of huge datasets. Very large models can be compiled in C for even faster simulation.

The Gaming simulation mode allows the user to step forward at discrete intervals and make changes to model variables at each step. Vensim can also be run over a network allowing multiple users to interact with a single model.

Vensim can use external data series as exogenous inputs to drive a model or to compare against data from simulation runs. You can create external data in text editors, or import from (or export to) database and spreadsheet applications.

Causal Tracing -- Causal Tracing is an advanced method of following the causes or uses of a variable (or its behavior) throughout a model. Model structure is traced with tree diagrams.

Model behavior is traced with strip graphs. Causal Tracing makes it far easier to thoroughly explore and debug a complex model.

Vensim’s unique approach to model analysis greatly speeds understanding of model behavior. A dataset stores the dynamic behavior of all variables in the model for later viewing and analysis.

Multiple simulations (experiments) can be performed and stored to allow comparison of behavior resulting from different conditions.

Reality Check -- Reality Check technology allows you to test simulation models against your understanding of reality. Constructing a model that accurately reflects a complex system can be very difficult.

Reality Checks provide an easy way of automatically checking statements about reality against behavior generated in a model, keeping the model in line with the real world.

Sensitivity Testing -- Sensitivity testing involves changing your assumptions about the value of inputs to the model while performing multiple simulations, then examining the uncertainty in selected output variables. Vensim’s Monte Carlo simulations, also known as 'Multivariate Sensitivity Simulations' automates sensitivity testing.

Output can be displayed as graphs with confidence bounds, individual simulation traces, or histograms. 'Latin Hypercube' sampling is also supported for faster sensitivity testing.

Subscripting (Arrays) -- Vensim features an advanced subscripting language for constructing very advanced 'arrayed models'. A simple model structure can be disaggregated to show detail complexity.

Each subscripted structure can be individually customized with different constants, multiple equations, subscript functions (such as summing over elements of a subscript), and up to eight (8) dimensions of subscripts. Multiple sub-ranges make it easy to construct and analyze subsets of an array.

Optimizing Models -- A Vensim model can be automatically calibrated to fit historical data series. You load the external data series and specify the parameters to adjust, and then Vensim automatically adjusts these parameters to get the best fit.

Vensim’s optimizing engine searches through complex multi- dimensional surfaces looking for optimal solutions. You define the payoff (to maximize or minimize) then choose the policy parameters to adjust. An efficient 'Powell hill climbing' algorithm searches for the best set of policy parameter values to maximize the payoff.

Custom Applications (Flight Simulators or Learning Environments) --

A Vensim Application or Venapp can be constructed to provide a management flight simulator interface for a model. Venapps allow individuals with little or No training in modeling to get meaningful access to a model.

Venapps are written in a simple scripting language. The Vensim Dynamic-Link Library (DLL) is a separate program that can be called from other applications such as Visual Basic, Delphi, Excel, and multimedia authoring tools.

Create your interface with these applications then call the DLL to access, simulate, and analyze a Vensim model. Vensim can also be set up as a Dynamic Data Exchange (DDE) server or client.

Molecules software --

Molecules are the building blocks of good system dynamics models. Molecules, and their organization, provide a framework for presenting important and commonly used elements of model structure to novice and experienced model builders.

By having access to tried and true formulations, modelers can review what has been done before and modify or directly incorporate these formulations into their own models. Molecules are now implemented in software, making it faster and easier for novice and experienced modelers to develop high quality system dynamics models.

Molecules -- Molecules are made of primitive stock and flow or auxiliary elements and are, in turn, the building blocks of complete models. A molecule is an element of substructure that serves a particular purpose.

The analogy with chemistry is Not exact, but the name does convey much of the same spirit. One of the simplest molecules, and one that probably appears in most models, is the decay process.

For a simple molecule such as this, it is probably easiest to simply check the form and enter the structure and equations directly. However, for more complicated molecules, it will be easier to incorporate molecules into a model and make the appropriate modifications to appearance, naming conventions, and equations.

Molecules are currently formulated as complete ‘dynamic models’. Since components of molecules may already be in place in a model, there needs to be flexibility on how much of the molecule to include. Portions of a molecule can be copied and incorporated into a model.

Molecules and Archetypes -- It is useful to distinguish between molecules and archetypes. Archetypes present dynamics lessons that have been learned from systems having certain structural characteristics.

Molecules are building blocks from which structure is created. Molecules improve the ability to represent structure, but do Not draw dynamic lessons from particular structures.

Software for Molecules -- The current implementation of molecules is an add-on to the Vensim® software. Ultimately, the molecule framework will be available as a stand-alone application that will allow users to look up, experiment with, and classify molecules in a number of different ways.

The molecules will be available for use with system dynamics software supporting the model interchange format (MIF) protocol. This will allow anyone with system dynamics software to use molecules.

The Vensim Software Family --

Vensim is available in several configurations to fit different modeling needs:

Vensim PLE (Personal Learning Edition) helps you get started with building system dynamics and systems thinking models. Vensim PLE is free for educational or personal use and can be downloaded from our web site.

Vensim Professional allows you to use subscripting for easy handling of detail complexity, contains a text editor, and has optimization capabilities including model calibration and policy optimization.

Vensim DSS enables you to create management flight simulators for models, to customize Vensim by defining macros or external functions, and to link to other programming software through the Vensim DLLs.

Included with Vensim are set of sample models. These demonstrate everything from simple physical systems to large and complex business and social systems. All configurations ship with a Model Reader program, which can be distributed for free, allowing others to review and simulate models.

Vensim PLE ships with a User's Guide. The other configurations ship with a Tutorial, a Modeling Guide, and a comprehensive Reference Manual. The complete documentation, including indexing, is also available through the on-line help system.

System 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 is curently in Beta and requires OSX version 10.4 or higher.

The Windows Version will run on the Macintosh using 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.


Manufacturer Web Site Vensim

Price Contact manufacturer.

G6G Abstract Number 20467

G6G Manufacturer Number 104093