Subject: Metaphors

Posted by Prof. Dr. Niall Palfreyman on 2/1/2006
In Reply To:Metaphors Posted by Richard Turnock on 1/27/2006

Message:

Ooh, I love these philosophical posts - thanks Richard. I'll try an initial definition which reflects my perception, and we'll see where I end up.

First of all, as I see it, the 1-stock-1-flow idea is an artificial abstraction which probably rarely coincides with a learner's model of the world. As such, I wouldn't normally talk about a feedback loop, but rather about a "transaction". So what do I mean by a transaction?

Instead of stocks and flows, I prefer to talk about "processes" which connect "stocks". To make the connection with SD, I'm basically thinking that a process is an agglomeration of flows and dependencies, and these flows and dependencies link with a number of different stocks. This entire constellation of a single process together with a set of linked stocks is what I mean by a transaction.

An example of a transaction is the coflow model on page of 499 of "Business Dynamics": two stocks "Capital Stock" and "Labour Requirements" interact via an agglomeration of flows and dependencies, which together I would call a process, and this process together with the two stocks I would call a transaction.

Physically, a transaction might be a salt dissociating into its composite ions, or two chemicals interacting with an enzyme to form a product, or it might be a fox population interacting with a rabbit population. In all of these cases it is clear to students that the interaction rate depends in general upon _all_ participating stocks in the transaction. The rate of dissociation of a salt depends for example upon the amount of salt present, but also on the amounts of the various ions present. The _process_ of dissociation brings together all these factors to create an ongoing change in the amounts of both salt and ions.

So, my definition of a transaction is fairly clear, at least to my chemistry-oriented students. On this basis it is then possible to arrive at the concept of a "feedback loop" by asking the question: "How does a population grow?" Here the population plays the role of both changer and changee - the process which leads to population growth must involve a flow to the population, but equally clearly, this process is dependent upon the current population level, so we have a flow from the process to the stock, and a dependency from the stock to the process. A "feedback loop" is simply a particularly simple case of a transaction involving only one stock.

As I say, this is just me thinking aloud, but I think I stand by my fundamental suggestion here: the "feedback loop" is an artefact of our attempt to analyse the transactions of nature down into components. And as such it will always be difficult for students to understand what we mean, as long as we neglect to introduce the idea via the concept of a transaction which corresponds much more closely to our experience of reality.

Best wishes,
Niall Palfreyman.