Vensim

Vensim is desktop software for developing, analysing and packaging dynamic feedback models, the kind of model known as system dynamics.² Such a model treats a system as stocks, quantities that build up or drain away, connected by flows and feedback loops; the picture above, from Ventana's site, shows one for the adoption of a product, drawn beside its result.¹ It is made by Ventana Systems of Harvard, Massachusetts. The company was formed in 1985, wrote Vensim as its own simulation language, and first released it publicly as version 1.50 in 1991.⁵ The current release is 10.4.0.²

There are four editions.^2,3 The Personal Learning Edition (PLE) is free for educational use and $50 for commercial use; PLE Plus adds data connections, Monte Carlo sensitivity runs (many runs with randomly varied inputs) and controls for building a simple user interface.^2,3 Professional, for large models, adds Causal Tracing of a model's structure and behaviour, optimisation, and subscripts (arrays).² DSS adds a tool for building management flight simulators, interactive games played on top of a model, along with compiled simulations.² Ventana emphasises calibration against data, and SyntheSim, which simulates fast enough to show results instantly.^2,5

Commercial prices per user are $169 for PLE Plus, $1,195 for Professional and $1,995 for DSS, with lower prices for public research and academic use.³ A licence is perpetual for the version bought and includes a year of updates.³ Ventana asks commercial and government users to buy a licence for anything beyond evaluation, including research with no educational component.⁴ Downloads are for Windows and macOS, and the download link is sent by email.⁴ The free Vensim Model Reader can be handed out with a model, including commercially, so others can run it without buying Vensim.^2,4

For someone modelling a place, Vensim suits questions about totals that change over years: how a town's housing stock, a reservoir or a fish population responds to a policy, and where the feedback loops push back. Models saved in Vensim's .mdl format can be run in Python with PySD, and the EMA Workbench can drive a Vensim model through many experiments.^6,7 For a similar tool, see Stella Architect.