Feedback is a process in which information about the past or the present influences the same phenomenon in the present or future. As part of a chain of cause-and-effect that forms a circuit or loop, the event is said to "feed back" into itself.
Ramaprasad (1983) defines feedback generally as "information about the gap between the actual level and the reference level of a system parameter which is used to alter the gap in some way", emphasising that the information by itself is not feedback unless translated into action.
"...'feedback' exists between two parts when each affects the other..."(p53)
Feedback is also a synonym for:
Feedback signal - the measurement of the actual level of the parameter of interest.
Feedback mechanism - the action or means used to subsequently modify the gap.
Feedback loop - the complete causal path that leads from the initial detection of the gap to the subsequent modification of the gap.
Self-regulating mechanisms have existed since antiquity, and the idea of feedback had started to enter economic theory in Britain by the eighteenth century, but it wasn't at that time recognized as a universal abstraction and so didn't have a name.
The verb phrase "to feed back", in the sense of returning to an earlier position in a mechanical process, was in use in the US by the 1860s, and in 1909, Nobel laureate Karl Ferdinand Braun used the term "feed-back" as a noun to refer to (undesired) coupling between components of an electronic circuit.
By the end of 1912, researchers using early electronic amplifiers (audions) had discovered that deliberately coupling part of the output signal back to the input circuit would boost the amplification (through regeneration), but would also cause the audion to howl or sing. This action of feeding back of the signal from output to input gave rise to the use of the term "feedback" as a distinct word by 1920.
There has been over the years some dispute as to the best definition of feedback. According to Ashby, mathematicians and theorists interested in the principles of feedback mechanisms prefer the definition of "circularity of action", which keeps the theory simple and consistent. For those with more practical aims, feedback should be a deliberate effect via some more tangible connexion.
"[Practical experimenters] object to the mathematician's definition, pointing out that this would force them to say that feedback was present in the ordinary pendulum ... between its position and its momentum - a 'feedback' that, from the practical point of view, is somewhat mystical. To this the mathematician retorts that if feedback is to be considered present only when there is an actual wire or nerve to represent it, then the theory becomes chaotic and riddled with irrelevancies."(p54)
Types of feedback
Main articles: negative feedback and positive feedback
Feedback is commonly divid
d into two types - usually termed positive and negative. The terms can be applied in two contexts:
the context of the gap between reference and actual values of a parameter, based on whether the gap is widening (positive) or narrowing (negative).
the context of the action or effect that alters the gap, based on whether it involves reward (positive) or non-reward/punishment (negative).
The two contexts may cause confusion, such as when an incentive (reward) is used to boost poor performance (narrow a gap). Referring to context 1, some authors use alternative terms, replacing 'positive/negative' with self-reinforcing/self-correcting, reinforcing/balancing, discrepancy-enhancing/discrepancy-reducing or regenerative/degenerative respectively. And within context 2, some authors advocate describing the action or effect as positive/negative reinforcement rather than feedback. Yet even within a single context an example of feedback can be called either positive or negative, depending on how values are measured or referenced. This confusion may arise because feedback can be used for either informational or motivational purposes, and often has both a qualitative and a quantitative component. As Connellan and Zemke (1993) put it:
"Quantitative feedback tells us how much and how many. Qualitative feedback tells us how good, bad or indifferent."(p102)
The terms "positive/negative" were first applied to feedback prior to WWII. The idea of positive feedback was already current in the 1920s with the introduction of the regenerative circuit. Friis and Jensen (1924) described regeneration in a set of electronic amplifiers as a case where the "feed-back" action is positive in contrast to negative feed-back action, which they mention only in passing. Harold Stephen Black's classic 1934 paper first details the use of negative feedback in electronic amplifiers. According to Black:
"Positive feed-back increases the gain of the amplifier, negative feed-back reduces it."
According to Mindell (2002) confusion in the terms arose shortly after this:
"...Friis and Jensen had made the same distinction Black used between 'positive feed-back' and 'negative feed-back', based not on the sign of the feedback itself but rather on its effect on the amplifierТs gain. In contrast, Nyquist and Bode, when they built on BlackТs work, referred to negative feedback as that with the sign reversed. Black had trouble convincing others of the utility of his invention in part because confusion existed over basic matters of definition."(p121)
Even prior to the terms being applied, James Clerk Maxwell had described several kinds of "component motions" associated with the centrifugal governors used in steam engines, distinguishing between those that lead to a continual increase in a disturbance or the amplitude of an oscillation, and those which lead to a decrease of the same.