Ventana software support forum

John Sterman provided an elegant answer to my feedback question. Thanks!

My first response was Duh! Of course! Then I pondered: why was there any
confusion on my part about this at all? I have read Forresters Intro,
Goodmans notes, and Richardson and Pugh; its not like I never saw this
before.

On the other hand, it is easy to consider feedback loops in terms of
setpoints, such as 37 deg for body temp. There is no setpoint after
death, but the ambient temp nevertheless consitutes the goal.

Obviously looking at the S&F diagram there is a connection from body temp
to the output rate, and the output rate obviously affects the temp.

I dont prefer causal loop diagrams, partly because sloppy versions of them
are used so frequently in medicine that they are worse than nothing. But
in this case I drew the simplest of all possible causal loop diagrams, and
of course, the clearest negative feedback loop in the world!

The most concise and clear statement possible was:

"Feedback exists anytime the rate of change in a state depends on the state
itself (either directly or indirectly, linearly or nonlinearly)."

Moving from body temp to drug elimination, we find that exactly the same
stock-and-flow diagram works for both (first-order, or exponential decay).
The more drug in the plasma, the more (constant fraction) is filtered into
the urine.

However, feedback has a certain meaning to most biologists.

(a) Exercise raises the body temp, which feeds back to produce sweating.

(b) Stress leads to hormone release (which then (i) decays exponentially).
However, the point of interest to most biologists is that (ii) the hormone
signal feeds back to inhibit its own release. Im guessing that most of
them will look at me with a quizical expression when I tell them that not
only is the classical feedback (ii) inherent in the system, but that (i)
is an example of feedback as well.

Most biologists would not consider self-limiting drug metabolism to be an
example of feedback.

I am not disagreeing with JS here at all; he is absolutely and
unequivocally correct. As always, Im trying to reconcile the thought
processes and culture of the biologist with the considerable experience and
literature in system dynamics.

Thanks John; your very clear and concise answer forced me right to the
heart of the matter.

ed
gallaher@teleport.com