Why walk when you can fly?
Vensim, the Ventana Simulation Environment, is an integrated framework for conceptualizing, building, simulating, analyzing, optimizing and deploying models of complex dynamic systems. Vensim has speed and efficiency so great, it's the difference between walking and flying.
Vensim uses a workbench-toolbox design that combines the simplicity of visual models with easy access to a host of powerful model simulation and analysis tools, yielding a spectacular increase in the speed of your work and the quality of your results.
A Family of Products
Vensim is available for both PC and Macintosh and comes in three different configurations so that you can choose the functionality you need. The standard Vensim allows you to create, articulate and simulate models—showing structure and simulating behavior. Vensim Professional adds more advanced modeling capabilities and enables optimization of model performance. Vensim DSS increases model capacity and provides tools for building decision support systems and learning simulators. Migration of models among configurations and platforms is seamless. Vensim offers the choices to fit your needs.
Vensim Helps Creativity
Vensim provides a flexible approach to creating models by allowing you to include ideas, build diagrams and, when appropriate, move into a formal simulation model. Your modeling can start with causal loop diagrams, equations, or stock and flow diagrams. Models can also be imported from other applications, providing the user with Vensim's powerful analysis and optimization tools. Vensim is designed to let you work the way you want to work.
Vensim is Powerful
Vensim makes it effortless and fun to move in and understand why a model is doing what it is doing. Powerful loop tracing tools, graphical comparisons, optimization tools and other utilities allow thorough analysis for in-depth understanding of complex systems. Vensim does the work necessary to make your life easier.
The ultimate tool for developing and analyzing dynamic feedback models
Vensim's standard configuration provides a powerful integrated environment for the development and analysis of complex dynamic feedback models.
Causal Loop Diagrams
Vensim makes building and annotating causal loop diagrams simple and straightforward. Just enter the concepts and connect them with arrows. Vensim maintains the connections, keeping structure while allowing you to rearrange and customize the diagram in many different ways.
Stock and Flow Diagrams
Vensim supports you in creating stock and flow diagramming to represent systems in greater structural detail. Stocks and flows can be mixed with causal loops to clearly distinguish the different system components.
Vensim allows you to build a number of different types of diagrams including flow charts, organizational charts and fishbone diagrams. Because Vensim keeps track of the concepts you enter and their connections, diagrams in different formats are kept logically consistent.
Vensim uses an intuitive workbench-toolbox design to let you analyze the models that you have built. The tools you want to use can be selected from the Toolbox and added to the Workbench arrayed along the left. The tools can be focused on a particular variable within the model, allowing identification of connected variables and feedback loops as well as the display of simulation results.
Diagrams must be turned into equation-based models in order to run quantitative simulations. Vensim facilitates the straightforward expression of mathematical relationships, such as:
profit = revenue - cost
Variables can be entered into the equation by clicking on an input, function or symbol, or by typing in the variables directly.
When you have finished writing an equation, Vensim checks it and records the relationships defined. If you enter an equation with a structure different from that on a diagram, the diagram will automatically be updated to reflect those changes.
Vensim has more than 50 built-in functions to choose from, including: test input patterns, a relational IF THEN ELSE, random number generators (binomial, exponential, gamma, normal, Poisson, triangular, Weibull), numerous delays, and standard scientific functions. In addition to the built-in functions, you can graphically define arbitrary functional relationships using Lookup tables.
Simulation and Analysis
Simulation is the process of taking the equations you have entered and using them to generate behavior. Vensim provides tools for displaying simulation results and analyzing and comparing simulation runs.
Vensim provides a simple mechanism for entering and recording changes to assumptions for comparing different runs. In gaming mode, simulation can be interrupted and restarted, with any inputs you make recorded to help in later analysis of model behavior.
Graphs and Tables
Vensim makes it easy for you to generate graphs and tables for simulation output. Because of the way Vensim stores data it is easy to compare results among different simulations of a model and among different models.
Vensim is designed to make extensive use of varied forms of data (time series, data with missing values, subjective data) both as inputs to models and for testing and validating model behavior. You can enter any number of data series, each with any number of arbitrarily spaced data points. Data from databases or other sources is easily incorporated into Vensim.
Data and simulation results can be viewed and used in the same way making it easy to compare simulations and historical data. Simulation results can also be exported for use in other applications.
Sensitivity Testing (requires Vensim PLE-Plus, Vensim Professional, or Vensim DSS)
After developing a model it is often desirable to know how sensitive the model is to different assumptions. While it is easy to test sensitivity by comparing a number of different simulations, this is often impractical when you need to test many parameters together.
Vensim has a sensitivity capability that makes it easy to run multivariate sensitivity simulations (Monte-Carlo) and Latin Hypercube simulations. Simply select the parameters that you want to test, and the results that you want to see. Vensim's sensitivity graph then will show you either simulation traces or confidence bounds as you choose.
Vensim provides you with a Causal Tracing capability that allows you to quickly determine the causes of behavior that you are seeing. By double clicking on the variable of interest, you can immediately see graphs for the causal inputs to that variable. Analysis that would take hours with other tools can be done in seconds with Vensim.
Reality Check is a powerful tool that allows you to test simulation models against your understanding of a problem. Reality Check uses your statements about conditions and the consequences that must follow, such as "if there are no workers, we do not produce any goods," or "if production is constant, sales cannot continue to grow." Reality Check tests your statements against behavior generated in the model, highlighting any violation.
Solving complex business problems
Vensim Professional adds to Vensim's standard configuration making it easier to develop, calibrate and optimize models for solving today's complex business problems.
In developing a model, the same piece of structure often will repeat itself a number of times. For example, you might wish to model a company with production facilities in several locations. It is possible to replicate the structure describing the production facilities, but subscripts offer a simpler solution. Using subscripts, one equation can be used to represent a large number of different facilities, regions, product lines and other attributes. Special subscript functions permit sums, products, minimums and maximums over all subscript values.
User Defined Macros
Often, you might find yourself repeating a series of equations for which it is not practical to use subscripts. For example, you might have a trend extrapolation computation that you want to use in several places. Vensim Professional allows you to write the equations with one macro definition, and then call that macro the same way you would call a regular Vensim function.
Tools and Reports
Vensim Professional adds a number of additional tools to be used in the analysis and display of model results. The Gantt chart as shown below
is useful for reviewing project performance under different scenarios. A Stats tool allows quick review of summary statistics for simulation results. The Bar Graph tool, by working across subscripts, makes it easy to compare conditions, such as total cost, at a point in time. Custom reports can be developed to mix text with simulation results in a convenient format.
In developing a simulation model, it is often helpful to adjust model parameters by comparing model behavior with historical data. Vensim Professional can automate this parameter adjustment process. You specify which data series you want to compare and which parameters you want to adjust, then Vensim automatically adjusts parameters to get the best fit.
After a model has been developed and calibrated it can be used to explore the policy choices available. Vensim Professional has an optimizer that automates policy exploration. You define the payoff that you would like to maximize, for example cumulative profits, along with the policy levers that you would like to adjust. Vensim will search the parameter space to find the best combination of policies.
Text-Based Model Building
In addition to diagram-based modeling, Vensim Professional has a Text Editor that allows you to enter equations directly facilitating many of the shortcuts a text editor can supply. With automatic cursor positioning for error correction and model analysis, the Text Editor can speed model development. You can enter equations for which a diagram does not seem necessary or modify equations developed in a diagram. As you make changes, Vensim Professional tracks the changes so that when you go back to diagrams, they are complete and up-to-date.
Creating decision support systems
Vensim DSS (Decision Support System) has features that build on the power of Vensim Professional, providing users with high-level development of models, including construction of interfaces that allow other users easy access to the models that you develop.
When developing a model, you might wish to include specialized functions, such as matrix inversion routine, that are not built-in. Vensim DSS allows you to write external functions into a Dynamic Link Library (DLL). These functions can be used like built-in Vensim functions, allowing you to create specialized models for specific needs not covered by the standard functions. Vensim DSS also enables you to make compiled simulations to speed the simulation process.
A Vensim application—or Venapp—combines models with a customized menu-driven screen that you design and build, providing an interface appropriate for your target audience. Venapps allow individuals with little or no training in modeling to get meaningful access to the results of a model.
Sketches can be displayed and critical assumptions emphasized to explain model structure. Functions that can be invoked and their purpose also can be described. The language and detail included can be made specific to your intended audience.
Almost all of the tools and functions that Vensim provides can be accessed from inside a Venapp. Games, executive information systems, management flight simulators and scenario builders no longer need to be a black box, but can be investigated through causal tracing and other analysis tools to provide understanding on what has happened and why. The diagram below illustrates a Venapp environment with buttons for choices and graphs displaying the results of model runs.
Venapps are a very natural extension of Vensim models, and can therefore be put together quite quickly. They are written in a simple scripting language, with workbench tools accessed by name.