Subject: Systems Language - Telling a Story

Posted by Warren Farr on 2/23/2010
In Reply To:Systems Language - Telling a Story Posted by Tim Joy on 2/23/2010

Message:

The Stella software handles the aggregate explanation of what is happening nicely. The genesis for any model formulation is a "dynamic hypothesis" that provides a top-level reasoning for why the observed behavior is happening. In this case: Why does the population grow to a limit and then (depending on model formulation) level out, or decline? Our top level dynamic hypothesis might be: As population grows, it uses more resources, so the more population, the less resources; further, the more resources, the more possible population. This can be represented as a CLD or in Stella using "sector frames" in the interface layer (see attachment: dynamic hypothesis 1). Note the polarity on the causal arrows.

The model, with more detail, resides underneath in the Stella model layer, where model components are assigned to one of the two sectors (either population or resource). See attachment: dynamic hypothesis 2. The division of model components into sectors highlights the important links in the model and effectively groups other details into "like categories".

This separates the "big picture" model message from the details. The message here is about growth to a limit. One of the details is how the population grows. The details in the example provided imply exponential growth, but these details could be swapped for the details of linear growth and the dynamic hypothesis would remain intact. Likewise, the resource could have a replenishment flow, but the dynamic hypothesis would again remain intact.

The model tells a more precise story, but the dynamic hypothesis abstracts the "major dynamics" and leaves the "minor dynamics" to be described modularly and when the class is ready. In addition, the ability to show the robustness of the "big picture" behavior regardless of detail (exponential or linear population growth) is a great abstraction which (hopefully) provides a deeper learning experience.

Warren Farr

Follow Ups:

Systems Language - Telling a Story - Tim Joy 2/24/2010