At the end of last week we had put together a conceptual model of our system and translated this model into STELLA language.  Our model was based on flow charting the system as we understood it. The thinking process is described on the initial flow chart. After talking with a consultant from Denise Johnston from High Performance Systems, the STELLA software company, we started to really understand the concepts of stocks and flows. This was a major change in our thinking about how this system works and especially about what stocks and flows are. 

At this point we changed our STELLA model to reflect our  understanding of how the system works.  On our first draft, we had created stocks based on concentrations; now we saw that the stocks should be based on quantities of material. We added stocks of water, plankton, and fish.  We used our concentration data to determine numbers of units of mercury in each mercury stock.  We made an assumption that the stocks of water, plankton, and fish were in the same numerical units in order to simplify our model.  Although this assumption may limit the usefulness of our model, we know that there are limitations in any model.   We decided that we were really interested in the human elements of the system,  in the impact of the polluter and in the effect on the victim of pollution . We decided to create a model which would tell us what would happen if pollution were reduced or eliminated over time.  We set out to gather the real data which would support our model. 

We put together a STELLA model of a very small part of the system.  With help from Denise, we realized that we would have to fully understand one piece at a time before we could create the entire model.  This model can be seen as an earlier development of the system. Go to Model #1