Feedback
Feedback is a process whereby some proportion of the output signal of a system is passed (fed back) to the input. This is often used to control the dynamic behavior of the system. Examples of feedback can be found in most complex systems, such as engineering, architecture, economics, thermodynamics, and biology. An example of a complex feedback system is the steering system of an automobile. While driving, a person receives signals from the environment, such as signs and hazards. The drivers brain processes the information and sends signals to the automobile via the steering wheel and pedals. The automobile responds by changing direction or speed accordingly.
Negative feedback was applied by Harold Stephen Black to electrical amplifiers in 1927, but he could not get his idea patented until 1937. Arturo Rosenblueth, a Mexican researcher and physician, co-authored a seminal 1943 paper Behavior, Purpose and Teleology that,  according to Norbert Wiener (another co-author of the paper), set the basis for the new science of cybernetics. Rosenblueth proposed that behavior controlled by negative feedback, whether in animal, human or machine, was a determinative, directive principle in nature and human creations.. This kind of feedback is studied in cybernetics and control theory.
In organizations, feedback is a process of sharing observations, concerns and suggestions between persons or divisions of the organization with an intention of improving both personal and organizational performance. Feedback can be unilateral, but experience suggests that bi-directional feedback is more effective in achieving continuous improvement in an organization.

# Overview
Feedback is both a mechanism, process and signal that is looped back to control a system within itself.  This loop is called the feedback loop.  A control system usually has input and output to the system; when the output of the system is fed back into the system as part of its input, it is called the "feedback."
Feedback and regulation are self related. The negative feedback helps to maintain stability in a system in spite of external changes. It is related to homeostasis. Positive feedback amplifies possibilities of divergences (evolution, change of goals); it is the condition to change, evolution, growth; it gives the system the ability to access new points of equilibrium.
For example, in an organism, most positive feedback provide for fast autoexcitation of elements of endocrine and nervous systems (in particular, in stress responses conditions) and play a key role in regulation of morphogenesis, growth, and development of organs, all processes which are in essence a rapid escape from the initial state. Homeostasis is especially visible in the nervous and endocrine systems when considered at organism level.

# Types of feedback
Types of feedback are:
- negative feedback: which tends to reduce output (but in amplifiers, stabilizes and linearizes operation),
- positive feedback: which tends to increase output, or
- bipolar feedback: which can either increase or decrease output.
Systems which include feedback are prone to hunting, which is oscillation of output resulting from improperly tuned inputs of first positive then negative feedback. Audio feedback typifies this form of oscillation.
Bipolar feedback is present in many natural and human systems. Feedback is usually bipolarthat is, positive and negativein natural environments, which, in their diversity, furnish synergic and antagonistic responses to the output of any system.

# Applications

## In biology
In biological systems such as organisms, ecosystems, or the biosphere, most parameters must stay under control within a narrow range around a certain optimal level under certain environmental conditions. The deviation of the optimal value of the controlled parameter can result from the changes in internal and external environments. A change of some of the environmental conditions may also require change of that range to change for the system to function. The value of the parameter to maintain is recorded by a reception system and conveyed to a regulation module via an information channel.
Biological systems contain many types of regulatory circuits, both positive and negative.  As in other contexts, Positive and negative don't imply consequences of the feedback have good or bad final effect. A negative feedback loop is one that tends to slow down a process, while the positive feedback loop tends to accelerate it.
The mirror neurons are part of a social feedback system, when an observed action is  mirrored  by the brain - like a self performed action.
Feedback is also central to the operations of genes and gene regulatory networks.  Repressor (see Lac repressor) and activator proteins are used to create genetic operons, which were identified by Francois Jacob and Jacques Monod in 1961 as feedback loops. These feedback loops may be positive
(as in the case of the coupling between a sugar molecule and the proteins that import sugar into a bacterial cell), or negative (as is often the case in metabolic consumption). Interactive models for interplay between positive and negative feedback loop in metabolism is presented
here.
Any self-regulating natural process involves feedback and is prone to hunting. A well known example in ecology is the oscillation of the population of snowshoe hares due to predation from lynxes.
In zymology, feedback serves as regulation of activity of an enzyme by its direct product(s) or downstream metabolite(s) in the metabolic pathway (see Allosteric regulation).
There is an ice-albedo positive feedback loop whereby melting snow exposes more dark ground (of lower albedo), which in turn absorbs heat and causes more snow to melt. This is part of the evidence of the danger of global warming.

## In economics and finance
A system prone to hunting (oscillating) is the stock market, which has both positive and negative feedback mechanisms. This is due to cognitive and emotional factors belonging to the field of behavioral finance. For example,
- When stocks are rising (a bull market), the belief that further rises are probable gives investors an incentive to buy (positive feedback, see also stock market bubble); but the increased price of the shares, and the knowledge that there must be a peak after which the market will fall, ends up deterring buyers (negative feedback).
- Once the market begins to fall regularly (a bear market), some investors may expect further losing days and refrain from buying (positive feedback), but others may buy because stocks become more and more of a bargain (negative feedback).
George Soros used the word "reflexism" to describe feedback in the financial markets and developed an investment theory based on this principle.
The conventional economic equilibrium model of supply and demand supports only ideal linear negative feedback and was heavily criticized by Paul Ormerod in his book "The Death of Economics" which in turn was criticized by traditional economists. This book was part of a change of perspective as economists started to recognise that Chaos Theory applied to nonlinear feedback systems including financial markets.

## In education
Young students will often look up to instructors as experts in the field and take to heart most of the things instructors say. Thus, it is believed that spending a fair amount of time and effort thinking about how to respond to students may be a worthwhile time investment. Here are some general types of feedback that can be used in many types of student assessment:
(Adapted from Flemming and Levie.)
A different application of feedback in education is the system for "continuous improvement" of engineering curricula monitored by the  .

## In electronic engineering
The processing and control of feedback is engineered into many electronic devices and may also be embedded in other technologies.
If the signal is inverted on its way round the control loop, the system is said to have negative feedback; otherwise, the feedback is said to be positive.   Negative feedback is often deliberately introduced to increase the stability and accuracy of a system.  This scheme can fail if the input changes faster than the system can respond to it.  When this happens, the lag in arrival of the feedback signal results in positive feedback, causing the output to oscillate or hunt Positive feedback is usually an unwanted consequence of system behaviour.
Harry Nyquist contributed the Nyquist plot for assessing the stability of feedback systems. An easier assessment, but less general, is based upon gain margin and phase margin using Bode plots (contributed by  Hendrik Bode). Design to insure stability often involves frequency compensation, one method of compensation being pole splitting.

## In control theory
Feedback is extensively used in control theory, using a variety of methods including state space (controls), pole placement and so forth.
The most common general-purpose controller using a control-loop feedback mechanism is a proportional-integral-derivative (PID) controller.  Each term of the PID controller copes with time.  The proportional term handles the present state of the system, the integral term handles its past, and the derivative or slope term tries to predict and handle the future.

## In mechanical engineering
In ancient times, the float valve was used to regulate the flow of water in Greek and Roman water clocks; similar float valves are used to regulate fuel in a carburetor and also used to regulate tank water level in the flush toilet.
The windmill was enhanced in 1745 by blacksmith Edmund Lee who added a fantail to keep the face of the windmill pointing into the wind. In 1787 Thomas Mead regulated the speed of rotation of a windmill by using a centrifugal pendulum to adjust the distance between the bedstone and the runner stone (i.e. to adjust the load).
The use of the centrifugal governor by James Watt in 1788 to regulate the speed of his steam engine was one factor leading to the Industrial Revolution. Steam engines also use float valves and pressure release valves as mechanical regulation devices. A mathematical analysis of Watt's governor was done by James Clerk Maxwell in 1868.
The Great Eastern was one of the largest steamships of its time and employed a steam powered rudder with feedback mechanism designed in 1866 by J.McFarlane Gray. Joseph Farcot coined the word servo in 1873 to describe steam powered steering systems. Hydraulic servos were later used to position guns. Elmer Ambrose Sperry of the Sperry Corporation designed the first autopilot in 1912. Nicolas Minorsky published a theoretical analysis of automatic ship steering in 1922 and described the PID controller.
Internal combustion engines of the late 20th century employed mechanical feedback mechanisms such as vacuum advance (see: Ignition timing) but mechanical feedback was replaced by electronic engine management systems once small, robust and powerful single-chip microcontrollers became affordable.

## In organizations
As an organization seeks to improve its performance, feedback helps it to make required adjustments.
Examples of feedback in organizations:
- Financial audit
- Performance appraisal
- Shareholders' meetings
- Marketing research
- 360-degree feedback
- Walkouts
- Lockouts

## In government
Examples of feedback in government are:
- Elections
- Mass media
- Revolution
- Curfews