building blocks for models

April 10, 2010

We’ve had a long debate within the field about the pros and cons of using standard generic structures to represent model elements. I tend to¬† be wary of their use, thinking that it’s important to approach a given modeling project with as few preconceptions as possible. But if social science and psychology are to be at all useful, they must present us with phenomena and explanations that apply across different cases. This suggests that there are indeed “generic structures” in the world. Let’s look at a few starting points. First, we look at the more heuristic approaches commonly called systems thinking, then at computer-simulation approaches.

Peter Senge’s book presented a set of “system archetypes” that boil down some commonly-found structures into causal loop diagrams and connect them heuristically to behavior over time. Archetypes are

images of common systemic situations. Each of these patterns occurs in a wide variety of domains, from ecology to economics to manufacturing; each offers its own strategic insights, and gives people a better picture of how the forces of the system may trap them.

System dynamics researchers have published descriptions of about a dozen archetypes. They include “Limits to Growth,” in which a seemingly boundless growth pattern runs up against unexpected limiting forces. (Total quality campaigns, for example, run up against institutional disappointment after the “low-hanging fruit” is picked.) In another archetype, “Shifting the Burden,” a more immediately inviting, short-term solution to a problem weakens the system’s ability to develop a more fundamental, but slower, approach.

source

To learn more about the idea in general as well as the specifics of the archetypes, take a look at the following resources. The Systems Thinker offers a brief introduction to the idea. For more details, take a look at Gene Bellinger’s overview of the archetypes and at this how-to document from the Society for Organizational Learning called Systems Archetypes Notes. On the Pegasus site, Michael Goodman and isee systems offer a detailed module on Systems Archetypes as part of an online course called Applying Systems Thinking and Common Archetypes to Organizational Issues. There’s also a recent series of blog posts tagged “archetype on the isee systems site.

How are archetypes used to address real-world challenges and improve performance? The Society for Organizational Learning and Pegasus are useful starting points. Here’s one case example from Applied Systems Thinking site: David Peter Stroh on Leveraging Change: The Power of Systems Thinking in Action. Take a look at the short paper presented there, and then consider: Which archetypes does he invoke in this case? If you’d like to see a second example applying systems thinking archetypes to a complex challenge, check out this study of Facilitating Reentry of Formerly Incarcerated People.

Going beyond causal diagramming, Jim Hines has assembled a very detailed and complete set that is excellently documented in this detailed publication, Molecules of Structure: Building Blocks for System Dynamics Models, version 2.2 (from around 2005).

Hines' molecules, from the Ventana Systems site

The molecules themselves are available from Ventana on the Vensim Molecules page.

Papers that discuss some of the issues entailed in coming up with generic archetypes include the two following:

Reinterpreting ‘generic structure’: evolution, application and limitations of a concept.
Lane, David C.; Smart, Chris
System Dynamics Review
1996 Volume 12 Issue 2, Pages 87 – 120

Abstract: This paper traces the evolution of the generic structure concept in system dynamics and discusses the different practical uses to which they have been put. A review of previous work leads to the identification of three different views of what a ‘generic structure’ is and, hence, what transferability means. These different views are distinguishable in application as well as in theory. Examination of these interpretations shows that the assumptions behind them are quite distinct. From this analysis it is argued that it is no longer useful to treat ‘generic structure’ as a single concept since the unity it implies is only superficial. The conclusion is that the concept needs unbundling so that different assumptions about transferability of structure can be made explicit, and the role of generic structures as generalisable theories of dynamic behaviour in system dynamics theory and practice can be debated and clarified more effectively.

Towards the definition and use of a core set of archetypal structures in system dynamics
Wolstenholme, Eric F.
System Dynamics Review
2003 Volume 19 Issue 1, Pages 7 – 26

There has been considerable research in the field of system dynamics over the last decade concerned with defining generic structures and templates by which to classify structures and behavioural insights in dynamic systems. These have appeared both in stock/flow and causal loop terms. This article focuses on generic causal loop structures more commonly known as system archetypes, a profusion of which have now been defined. The purpose of the article is to improve the usefulness of system archetypes both as free standing devices to aid model conceptualisation and as a means of disseminating insights arising from models. In order to achieve this three postulates are made: First, that system archetypes can be usefully condensed down to a more understandable core set of four totally generic archetypes, consisting of the four ways of ordering a pair of reinforcing and balancing feedback loops. These are described, named and current archetypes mapped onto them with specific examples. Second, that for every problem archetype there exists a closed loop solution archetype. It is suggested that some misunderstandings with current archetypes arise from the fact that they often combine problem and solution links in the same diagram. Third, that each archetype has important characteristics, which are vital to understanding the role of archetypes in assisting systemic thinking. The particular characteristic introduced and highlighted is the concept of organisational boundaries. The article concludes by discussing the importance of the reduced set of archetypes, and organisational boundaries in particular, in explaining some barriers to implementing systemic thinking, using a range of examples encountered by the author in recent consulting practice.

Finally, where do I come down on the issues? I think that it’s really important not to reinvent new model structures when we already have much to build on. To that end, we put together a spreadsheet listing all the model elements presented in John Sterman’s business dynamics book as an aid in your search for relevant models. My students can download this from the class site and we hope to make it available to others soon.

Leave a Comment

Previous post:

Next post: