Subject: Is this a causal loop?

Posted by John Heinbokel on 2/27/2004
In Reply To:Is this a causal loop? Posted by Della Robertson on 1/27/2004

Message:

Mornin' Della,

Coupla' quick thoughts:

1. The CLE has some wonderful materials that focus on CLDs. The "Tips"
sheets put together by Catalina Foothills are very helpful; see url below.
Also in the CLE collection is the set of "rubrics" developed to support the Dynamiquest program. Search CLE for "rubrics" to track that one down (I'm getting lazy in my old age).

http://www.clexchange.org/ftp/documents/Implementation/IM2003-12TipsUsingSDT
ools.pdf

2. One thing to keep a focus on is that these are CAUSAL as well as loops.
In your first example, I can see how the second element (books being brought in -- a flow) causes the third element (book in library -- a stock) to change (grow). What is quite unclear to me is the logic of saying that the number of books in the library will cause the books being brought in to change. I can see some possible stories, e.g. once all the books the library owns are back on the shelves, nothing will be left to bring back, but if there is a reasonably logical story, it should be built more specifically/explicitly so that I, as reader, don't have to infer the looper's mental model from my own logic or experience.

3. In the second 'loop,' the use of the element "rainforest" is problematic. That, per se, is not easily or unambigously quantified. We can think of a large or a small rainforest, but more useful and explicit would be acres (or hectares0 of rainforest. If that's the meaning that's meant (and again, it's not good practise to force others to create that meaning)then I have the same problem I had with the first. Yes changing the size of rainforest influences could easily cause the (number of?) bird species to change. So far so good, but what is the causal mechanism that would lead a changed number of bird species to change the size of the rainforest (the link that 'closes the loop')?

I'm sure those three points don't exhaust the possibilities, so I'll be curious to see how others contribute/react.

Hope to see you in Skamania this summer, Della. Think we will?

john heinbokel
center for system dynamics
vermont commons school