System dynamicists use the phrase "feedback loop" to stand for all manner
of circular causal loops. It is fair to say that system dynamicists
probably do not have a definition for the word "feedback" by itself. It
is the loop we label.
Historically, that usage is not universal. Von Bertalanffy, for example,
objected to using the term feedback in such an all-encompassing way and
preferred to reserve it for the deliberate "feeding back" of information
in a system, probably for control pruposes. So he objected to thinking
about the structure of, say, a predator-prey system as a feedback system,
because all the loops were naturally occurring -- none involved the
deliberate "feeding back" of information by actors or managers in the system.
Ashby was comfortable with a usage like ours, but he got confused by the
discrete "loop" x(t) = .5*y(t-1), y(t) = .5*x(t-1), which he concluded
was a goal-seeking positive feedback loop, and that was so bizarre to him
he concluded people should stop using the feedback concept.
The Macy meetings in the 1940s and 50s, which began the field of
cybernetics, adopted the definition that feedback was "the alteration of
input by output." That definition is much like the engineering one
sent to this list. It is unlike the way a system dynamicist thinks about
it because it focuses on ideas like input and output, which do not have
much meaning in an endogenous, circular causal system. Since were
dealing with a fundamentally endogenous perspective, we think about
"feedback loops" but not about "feedback" by itself.
Thus we dont think much about "feedforward" either. If it has a real
meaning in an endogenous circular causal system, it would appear as just
another link or set of links which add to the causal complexity of the
system. I could imagine that restructuring the beer game, for example,
so that the order stream at the customer end is feed to the other
positions in the game could be thought of as a kind of "feedforward" in a
new control system designed to eliminate the oscillations in the game.
But system dynamicists would probably refer to this as a restructuring of
the information system and would not really know whether the restructuring
was "forward" or "back."
One can find definitions of feedback loops and the feedback perspective
in various system dynamics writings, notably the early writings of
Forrester. Youll also find much discussion of this in Feedback
Thought. I became very interested during that research to see the number
of different ways the idea(s) was discussed. There are serious
differences in emphasis in the various definitions, which have serious
implications for practice, which apparently depend on the literature and
contacts one has access to.
...GPR
-----------------------------------------------------------------------------
George P. Richardson
G.P.Richardson@Albany.edu
Rockefeller College of Public Affairs and Policy Phone: 518-442-3859
University at Albany - SUNY, Albany, NY 12222 Fax: 518-442-3398
-----------------------------------------------------------------------------