Stella Software System Dynamics

[Edco Haglund]

STELLAshort for Systems Thinking, Experimental Learning Laboratory with Animation; also marketed as iThink) is a visual programming language for system dynamics modeling introduced by Barry Richmond in 1985. The program, distributed by isee systems (formerly High Performance Systems) allows users to run models created as graphical representations of a system using four fundamental building blocks. STELLA has been used in academia as a teaching tool and has been utilized in a variety of research and business applications. The program has received positive reviews, being praised in particular for its ease of use and low cost.[3][4][5]

While working at the Massachusetts Institute of Technology in the 1960s, Jay Wright Forrester developed the earliest understanding of system dynamics which he argued could only be understood using models.[6] Dartmouth College systems science professor Barry Richmond founded High Performance Systems in 1984.[7] With the financial support of Analog Devices, Inc. and technical support from Apple Computer, he developed STELLA (short for Structural Thinking, Experimental Learning Laboratory with Animation) at his company.[8][9] He presented the prototype for the visual programming language in 1985 at the System Dynamics Society's annual conference in a paper entitled "STELLA: Software for Bringing System Dynamics to the Other 98%".[4][8][10]

STELLA's approach to modeling systems shares some similarities with a precursor, the DYNAMO simulation language. DYNAMO explicitly defined "stocks" (reservoirs) and "flows" (inputs and outputs) as key variables in a system, a vocabulary that STELLA shares.[9] Within STELLA, users are presented with a graphical user interface in which they may create graphical models of a system using four fundamentals: stocks, flows, converters, and connectors.[13] Relationships between converters (which convey transforming variables) and other elements may be drawn with converters. Users are able to input values for stocks, flows, and converters (including a variety of built-in functions).[14] STELLA does not differentiate between external and intermediate variables within a system; all of them are represented with converters.[15]

STELLA runs one window at a time, meaning that only one model can be run at any given moment.[4] The program's native file formats are denoted either by an .stm, .stmx, .itm, or .itmx filename extension. STELLA also uses the emerging XML-based standard for storing models, XMILE.[19] In 2012, two researchers released StellaR, software which can translate STELLA models into the R programming language.[20]

Because of its simplicity relative to more complex modeling languages, STELLA has been cited as a useful tool in educational settings.[6] Richmond derisively viewed most education as "assimilating content" and proposed systems thinking as a remedy to this.[22]

In 1987, High Performance Systems released a guide to STELLA encouraging its use in academic settings[23] and numerous textbooks have been published that teach modeling and systems thinking using the software.[24][25] Sample exercises with STELLA include recreating the Daisyworld model,[26] simulating the Easter Island population crash,[27] and modeling the protagonist's motivation throughout William Shakespeare's Hamlet.[28]

A 2010 study of the efficacy of project-based learning upon a watershed-modeling project undertaken by 72 middle schoolers found that the addition of a STELLA modeling component in the project improved overall comprehension of the material over traditional methods, especially among female students who outperformed their male counterparts with the addition of STELLA.[29]

The software is also used in research settings. Among other projects, researchers have used STELLA to apply Hubbert peak theory to the Chinese coal supply,[30] to model atrazine dynamics within agricultural lands,[31] and to simulate the interactions between marine macroinvertebrates.[32]

isee systems[a] markets an identical software targeted at business consumers under the name iThink (previously STELLA for Business).[5][33] iThink models have been applied to a variety of systems including manufacturing lines,[13] hospital waste in developing nations,[34] coordination between an emergency room and hospital beds,[35] and competition in the home video market.[5]

Writing for Complexity in 1997, Benedikt Hallgrmsson found the program's accompanying manual to be overzealous in its promotion of systems theory but cautioned that "the manual need not detract from what is otherwise a very well-thought-out and constructed program."[36]

System dynamics software (such as Stella, iThink, Vensim, and Powersim) is based on the standard stock and flow approach developed by Professor Jay W. Forrester at MIT in the late 1950s and early 1960s. Models based on system dynamics are built using three principal element types (stocks, flows, and converters), and put emphasis on understanding the feedback structure of systems. System dynamics software packages are typically used for simulating business and organizational systems and simple engineering and scientific systems.

Although GoldSim is similar to system dynamics programs in many ways (and can simulate any kind of system that these tools can), GoldSim moves beyond the relatively restrictive stock and flow syntax in order to more realistically model complex systems. In particular, GoldSim differs from most system dynamics software packages by offering the following features:

In June, isee systems and IBM sponsored a new technical committee in OASIS, a large standards organization. This committee is developing a new system dynamics modeling standard called XMILE. This blog post will answer some important questions about XMILE.

XMILE is the XML representation of a system dynamics model. SMILE is the underlying system dynamics language that is represented in XML using XMILE. In this way, it is very similar to the DYNAMO language originally used to create system dynamics models. SMILE could eventually be encoded using something other than XML.

All of the isee systems products (version 10 and later) already use the XMILE standard in its draft form. As the standard evolves, isee systems products will be updated to meet the changing standard and your models will be translated forward so they remain XMILE-compatible

XMILE focuses on the language of classic system dynamics, rooted in DYNAMO. While we anticipate the language to expand to include both discrete simulation and agent-based modeling, version one of the XMILE specification is restricted to system dynamics modeling.

XMILE is used to describe the model and is the format used for saving it. A model snippet is shown below with the XMILE that completely describes both its simulation and its drawing properties (in the display tag).

The structure is actually separate from the layout in the XML file. All visual information is embedded within display tags and can be ignored. XMILE defines three separate levels of compliance, with the lowest level being simulation information only (i.e., structure). A model does not need to include display information and any application is free to ignore it.

Stella II is an accessible object-oriented programming environment that can be used to introduce students to the basics of computational biogeochemical models. It utilizes Stella, a visual programming language for system dynamics modeling.

This blog provides an overview of system exits, local performance on this metric, and highlights opportunities for performance improvement within the Charlotte-Mecklenburg homeless services system.

Days Homeless, Exits, Returns: Reviews system level performance for number of days homeless, permanent housing exits from the homeless system, and returns to homelessness after exits to a permanent destination. Each measure can be examined by pathway (combination of projects types a household uses when moving through the homelessness system), population group, and race and ethnicity.

Demographics: Provides an overview of demographic characteristics for households and individuals served by household type and project type and a comparison of demographic characteristics by project type.

The Stella P System Exits module consists of four components: Exits Overview, Exits by Pathway, Exits by Population, and Exits by Race and Ethnicity. This blog will examine the first three. A future blog will focus on how Stella P can be used to assess system performance by race and ethnicity.

The system exits metric is based on homeless system exits which are defined as the last exit from an HMIS participating project in the reporting period with no enrollment in another project type for at least 14 days following the exit. If a household enrolls in another project type within 14 days, the exit is considered a project exit but not a system exit because the household did not exit the homeless services system, they only exited a particular project. The household is not considered to have exited the system until they exit a project and do not enroll in another project prior to the 14-day cutoff. As long as a household remains enrolled in an emergency shelter, safe haven, transitional housing, or rapid rehousing or permanent supportive housing (even if housed) within the homeless services system, they are not considered to have exited the system.

The key performance metric for exits is exits to a permanent destination. Permanent destinations include exits to home ownership or rental with or without subsidy and staying with friends or family permanently. The system exits overview page shows an overview of permanent exits from the homeless services system by household type, exits by destination type, and trends in the number of exits to a permanent destination in the previous three years. The exits by pathway page shows exits by the combination of projects that a household is served in during their experience in the homeless services system. The exits by population group page shows the percentage of each population group and the percentages of each exit type by population. Both the exits by pathway and exits by population group pages can be used to drill down to specific household and exit destination types.

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