Common cause and special cause - AbsoluteAstronomy.com

Type of variation	Synonyms
Common cause	Chance cause Non-assignable cause Noise Natural pattern
Special cause	Assignable cause Signal Unnatural pattern

Common- and special-causes are the two distinct origins of variation in a process, as defined in the statistical

Statistics

Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....

thinking and methods of Walter A. Shewhart

Walter A. Shewhart

Walter Andrew Shewhart March 18, 1891 - March 11, 1967) was an American physicist, engineer and statistician, sometimes known as the father of statistical quality control.W...

and W. Edwards Deming

W. Edwards Deming

William Edwards Deming was an American statistician, professor, author, lecturer and consultant. He is perhaps best known for his work in Japan...

. Briefly, "common-cause" is the usual, historical, quantifiable variation in a system, while "special-causes" are unusual, not previously observed, non-quantifiable variation.

The distinction is fundamental in philosophy of statistics

Philosophy of statistics

The philosophy of statistics involves the meaning, justification, utility, use and abuse of statistics and its methodology, and ethical and epistemological issues involved in the consideration of choice and interpretation of data and methods of Statistics....

and philosophy of probability, with different treatment of these issues being a classic issue of probability interpretations

Probability interpretations

The word probability has been used in a variety of ways since it was first coined in relation to games of chance. Does probability measure the real, physical tendency of something to occur, or is it just a measure of how strongly one believes it will occur? In answering such questions, we...

, being recognised and discussed as early as 1703 by Gottfried Leibniz

Gottfried Leibniz

Gottfried Wilhelm Leibniz was a German philosopher and mathematician. He wrote in different languages, primarily in Latin , French and German ....

; various alternative names have been used over the years.

The distinction has been particularly important in the thinking of economist

Economist

An economist is a professional in the social science discipline of economics. The individual may also study, develop, and apply theories and concepts from economics and write about economic policy...

s Frank Knight

Frank Knight

Frank Hyneman Knight was an American economist who spent most of his career at the University of Chicago, where he became one of the founders of the Chicago school. Nobel laureates James M. Buchanan, Milton Friedman and George Stigler were all students of Knight at Chicago. Knight supervised...

, John Maynard Keynes

John Maynard Keynes

John Maynard Keynes, Baron Keynes of Tilton, CB FBA , was a British economist whose ideas have profoundly affected the theory and practice of modern macroeconomics, as well as the economic policies of governments...

and G. L. S. Shackle

G. L. S. Shackle

George Lennox Sharman Shackle was an English economist. He made a practical attempt to challenge classical rational choice theory and has been characterised as a "post-Keynesian," though he is influenced as well by Austrian economics; he has been described as drawing "Keynesian conclusions from...

Origins and concepts

In 1703, Jacob Bernoulli wrote to Gottfried Leibniz

Gottfried Leibniz

Gottfried Wilhelm Leibniz was a German philosopher and mathematician. He wrote in different languages, primarily in Latin , French and German ....

to discuss their shared interest in applying mathematics

Mathematics

Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...

and probability

Probability

Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...

to games of chance. Bernoulli speculated whether it would be possible to gather mortality

Mortality rate

Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...

data from gravestones and thereby calculate, by their existing practice, the probability of a man currently aged 20 years outliving a man aged 60 years. Leibniz

Gottfried Leibniz

Gottfried Wilhelm Leibniz was a German philosopher and mathematician. He wrote in different languages, primarily in Latin , French and German ....

replied that he doubted this was possible as:

Nature has established patterns originating in the return of events but only for the most part. New illnesses flood the human race, so that no matter how many experiments you have done on corpses, you have not thereby imposed a limit on the nature of events so that in the future they could not vary.

This captures the central idea that some variation is predictable, at least approximately in frequency. This common-cause variation is evident from the experience base. However, new, unanticipated, emergent or previously neglected phenomena (e.g. "new diseases") result in variation outside the historical experience base. Shewhart

Walter A. Shewhart

Walter Andrew Shewhart March 18, 1891 - March 11, 1967) was an American physicist, engineer and statistician, sometimes known as the father of statistical quality control.W...

and Deming

W. Edwards Deming

William Edwards Deming was an American statistician, professor, author, lecturer and consultant. He is perhaps best known for his work in Japan...

argued that such special-cause variation is fundamentally unpredictable in frequency of occurrence or in severity.

John Maynard Keynes

John Maynard Keynes

emphasised the importance of special-cause variation when he wrote:

By “uncertain” knowledge ... I do not mean merely to distinguish what is known for certain from what is only probable. The game of roulette is not subject, in this sense, to uncertainty ... The sense in which I am using the term is that in which the prospect of a European war is uncertain, or the price of copper and the rate of interest twenty years hence, or the obsolescence of a new invention ... About these matters there is no scientific basis on which to form any calculable probability whatever. We simply do not know!

Common-cause variation

Common-cause variation is characterised by:

Phenomena constantly active within the system;
Variation predictable probabilistically
Probability
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...

;
Irregular variation within an historical experience base; and
Lack of significance in individual high or low values.

The outcomes of a perfectly balanced roulette

Roulette

Roulette is a casino game named after a French diminutive for little wheel. In the game, players may choose to place bets on either a single number or a range of numbers, the colors red or black, or whether the number is odd or even....

wheel are a good example of common-cause variation. Common-cause variation is the noise within the system.

Walter A. Shewhart

Walter A. Shewhart

Walter Andrew Shewhart March 18, 1891 - March 11, 1967) was an American physicist, engineer and statistician, sometimes known as the father of statistical quality control.W...

originally used the term chance-cause. The term common-cause was coined by Harry Alpert in 1947. The Western Electric Company used the term natural pattern. Shewhart called a process that features only common-cause variation as being in statistical control. This term is deprecated by some modern statisticians who prefer the phrase stable and predictable.

Special-cause variation

Special-cause variation is characterised by:

New, unanticipated, emergent or previously neglected phenomena within the system;
Variation inherently unpredictable, even probabilistically;
Variation outside the historical experience base; and
Evidence of some inherent change in the system or our knowledge of it.

Special-cause variation always arrives as a surprise. It is the signal within a system.

Walter A. Shewhart originally used the term assignable-cause. The term special-cause was coined by W. Edwards Deming

W. Edwards Deming

William Edwards Deming was an American statistician, professor, author, lecturer and consultant. He is perhaps best known for his work in Japan...

. The Western Electric Company used the term unnatural pattern.

Common causes

Inappropriate procedures
Poor design
Poor maintenance of machines
Lack of clearly defined standing operating procedure
Standing operating procedure
The terms standard operating procedure or SOP, is used in a variety of different contexts, such as healthcare, education, industry or the military. The military uses the term Standing Operating Procedure- rather than Standard- because an SOP refers to an organization's unique procedures, which are...

s
Poor working conditions, e.g. lighting, noise, dirt, temperature, ventilation
Substandard raw materials
Assurement error
Quality control
Quality control
Quality control, or QC for short, is a process by which entities review the quality of all factors involved in production. This approach places an emphasis on three aspects:...

error
Vibration in industrial processes
Ambient temperature and humidity
Normal wear and tear
Variability in settings
Computer response time

Special causes

Poor adjustment of equipment
Operator falls asleep
Faulty controllers
Machine malfunction
Computer crashes
Poor batch of raw material
Power surges
High healthcare demand from elderly people
Broken part
Abnormal traffic (click-fraud) on web ads
Extremely long lab testing turnover time due to switching to a new computer system
Operator absent

Importance to economics

In economics, this circle of ideas is referred to under the rubric of "Knightian uncertainty

Knightian uncertainty

In economics, Knightian uncertainty is risk that is immeasurable, not possible to calculate.Knightian uncertainty is named after University of Chicago economist Frank Knight , who distinguished risk and uncertainty in his work Risk, Uncertainty, and Profit:- Common-cause and special-cause :The...

". John Maynard Keynes

John Maynard Keynes

and Frank Knight

Frank Knight

both discussed the inherent unpredictability of economic systems in their work and used it to criticise the mathematical approach to economics

Economics

Economics is the social science that analyzes the production, distribution, and consumption of goods and services. The term economics comes from the Ancient Greek from + , hence "rules of the house"...

, in terms of expected utility

Utility

In economics, utility is a measure of customer satisfaction, referring to the total satisfaction received by a consumer from consuming a good or service....

, developed by Ludwig von Mises

Ludwig von Mises

Ludwig Heinrich Edler von Mises was an Austrian economist, philosopher, and classical liberal who had a significant influence on the modern Libertarian movement and the "Austrian School" of economic thought.-Biography:-Early life:...

and others. Keynes in particular argued that economic systems did not automatically tend to the equilibrium of full employment

Full employment

In macroeconomics, full employment is a condition of the national economy, where all or nearly all persons willing and able to work at the prevailing wages and working conditions are able to do so....

owing to their agents' inability to predict the future. As he remarked in The General Theory of Employment, Interest and Money:

... as living and moving beings, we are forced to act ... [even when] our existing knowledge does not provide a sufficient basis for a calculated mathematical expectation.

Keynes's thinking was at odds with the classical liberalism

Classical liberalism

Classical liberalism is the philosophy committed to the ideal of limited government, constitutionalism, rule of law, due process, and liberty of individuals including freedom of religion, speech, press, assembly, and free markets....

of the Austrian school

Austrian School

The Austrian School of economics is a heterodox school of economic thought. It advocates methodological individualism in interpreting economic developments , the theory that money is non-neutral, the theory that the capital structure of economies consists of heterogeneous goods that have...

of economist

Economist

An economist is a professional in the social science discipline of economics. The individual may also study, develop, and apply theories and concepts from economics and write about economic policy...

s, but G. L. S. Shackle

G. L. S. Shackle

recognised the importance of Keynes's insight and sought to formalise it within a free-market philosophy.

In financial economics

Financial economics

Financial Economics is the branch of economics concerned with "the allocation and deployment of economic resources, both spatially and across time, in an uncertain environment"....

, the black swan theory

Black swan theory

The black swan theory or theory of black swan events is a metaphor that encapsulates the concept that The event is a surprise and has a major impact...

of Nassim Nicholas Taleb is based on the significance and unpredictability of special-causes.

Importance to industrial and quality management

A special-cause failure is a failure that can be corrected by changing a component or process, whereas a common-cause failure is equivalent to noise in the system and specific actions cannot be made to prevent the failure.

Harry Alpert observed:

A riot occurs in a certain prison. Officials and sociologists turn out a detailed report about the prison, with a full explanation of why and how it happened here, ignoring the fact that the causes were common to a majority of prisons, and that the riot could have happened anywhere.

The quote recognises that there is a temptation to react to an extreme outcome and to see it as significant, even where its causes are common to many situations and the distinctive circumstances surrounding its occurrence, the results of mere chance. Such behaviour has many implications within management, often leading to ad-hoc interventions that merely increase the level of variation and frequency of undesirable outcomes.

Deming

W. Edwards Deming

William Edwards Deming was an American statistician, professor, author, lecturer and consultant. He is perhaps best known for his work in Japan...

and Shewhart

Walter A. Shewhart

Walter Andrew Shewhart March 18, 1891 - March 11, 1967) was an American physicist, engineer and statistician, sometimes known as the father of statistical quality control.W...

both advocated the control chart

Control chart

Control charts, also known as Shewhart charts or process-behaviour charts, in statistical process control are tools used to determine whether or not a manufacturing or business process is in a state of statistical control.- Overview :...

as a means of managing a business process

Business process

A business process or business method is a collection of related, structured activities or tasks that produce a specific service or product for a particular customer or customers...

in an economically efficient manner.

Deming and Shewhart

Within the frequency probability

Frequency probability

Frequency probability is the interpretation of probability that defines an event's probability as the limit of its relative frequency in a large number of trials. The development of the frequentist account was motivated by the problems and paradoxes of the previously dominant viewpoint, the...

framework, there is no process whereby a probability

Probability

can be attached to the future occurrence of special cause. One might naively ask whether the Bayesian

Bayesian probability

Bayesian probability is one of the different interpretations of the concept of probability and belongs to the category of evidential probabilities. The Bayesian interpretation of probability can be seen as an extension of logic that enables reasoning with propositions, whose truth or falsity is...

approach does allow such a probability to be specified. The existence of special-cause variation led Keynes

John Maynard Keynes

and Deming

W. Edwards Deming

William Edwards Deming was an American statistician, professor, author, lecturer and consultant. He is perhaps best known for his work in Japan...

to an interest in bayesian probability

Bayesian probability

, but no formal synthesis emerged from their work. Most statisticians of the Shewhart-Deming school take the view that special causes are not embedded in either experience or in current thinking (that's why they come as a surprise; their prior probability has been neglected—in effect, assigned the value zero) so that any subjective probability is doomed to be hopelessly badly calibrated in practice.

It is immediately apparent from the Leibniz

Gottfried Leibniz

Gottfried Wilhelm Leibniz was a German philosopher and mathematician. He wrote in different languages, primarily in Latin , French and German ....

quote above that there are implications for sampling

Sampling (statistics)

In statistics and survey methodology, sampling is concerned with the selection of a subset of individuals from within a population to estimate characteristics of the whole population....

. Deming observed that in any forecasting activity, the population is that of future events while the sampling frame

Sampling frame

In statistics, a sampling frame is the source material or device from which a sample is drawn. It is a list of all those within a population who can be sampled, and may include individuals, households or institutions....

is, inevitably, some subset

Subset

In mathematics, especially in set theory, a set A is a subset of a set B if A is "contained" inside B. A and B may coincide. The relationship of one set being a subset of another is called inclusion or sometimes containment...

of historical events. Deming held that the disjoint nature of population and sampling frame was inherently problematic once the existence of special-cause variation was admitted, rejecting the general use of probability and conventional statistics

Statistics

in such
situations. He articulated the difficulty as the distinction between analytic and enumerative statistical studies

Analytic and enumerative statistical studies

Analytic and enumerative statistical studies are two types of scientific studies:In any statistical study the ultimate aim is to provide a rational basis for action. Enumerative and analytic studies differ by where the action is taken...

.

Shewhart

Walter A. Shewhart

Walter Andrew Shewhart March 18, 1891 - March 11, 1967) was an American physicist, engineer and statistician, sometimes known as the father of statistical quality control.W...

argued that, as processes subject to special-cause variation were inherently unpredictable, the usual techniques of probability could not be used to separate special-cause from common-cause variation. He developed the control chart

Control chart

as a statistical heuristic

Heuristic

Heuristic refers to experience-based techniques for problem solving, learning, and discovery. Heuristic methods are used to speed up the process of finding a satisfactory solution, where an exhaustive search is impractical...

to distinguish the two types of variation. Both Deming and Shewhart

Walter A. Shewhart

Walter Andrew Shewhart March 18, 1891 - March 11, 1967) was an American physicist, engineer and statistician, sometimes known as the father of statistical quality control.W...

advocated the control chart as a means of assessing a process's state of statistical control and as a foundation for forecasting.

Keynes

John Maynard Keynes

identified three domains of probability:

Frequency probability;
Subjective or Bayesian probability; and
Events lying outside the possibility of any description in terms of probability (special causes)

- and sought to base a probability theory

Probability theory

Probability theory is the branch of mathematics concerned with analysis of random phenomena. The central objects of probability theory are random variables, stochastic processes, and events: mathematical abstractions of non-deterministic events or measured quantities that may either be single...

thereon.

In engineering

Common mode, or common cause, failure has a more specific meaning in engineering. It refers to events which are not statistically independent

Statistics

. That is, failures in multiple parts of a system caused by a single fault, particularly random failures due to environmental conditions or aging. An example is when all of the pumps for a fire sprinkler system are located in one room. If the room becomes too hot for the pumps to operate, they will all fail at essentially the same time, from one cause (the heat in the room).

Another example is an electronic system wherein a fault in a power supply injects noise onto a supply line, causing failures in multiple subsystems.

This is particularly important in safety-critical systems using multiple redundant

Redundancy (engineering)

In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....

channels. If the probability of failure in one subsystem is p, then it would expected that an N channel system would have a probability of failure of p^N. However, in practice, the probability of failure is much higher because they are not statistically independent; for example ionizing radiation

Ionizing radiation

Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

or electromagnetic interference

Electromagnetic interference

Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...

(EMI) may affect both channels.

The principle of redundancy states that, when events of failure of a component are statistically independent, the probabilities of their joint occurrence multiply. Thus, for instance, if the probability of failure of a component of a system is one in one thousand per year, the probability of the joint failure of two of them is one in one million per year, provided that the two events are statistically independent. This principle favors the strategy of the redundancy of components. One place this strategy is implemented is in RAID 1, where two hard disks store a computer's data redundantly.

But even so there can be many common modes: consider a RAID1 where two disks are purchased online and are installed in a computer, there can be many common modes:

The disks are likely to be from the same manufacturer and of the same model, therefore they share the same design flaws.
The disks are likely to have similar serial numbers, thus they may share any manufacturing flaws affecting production of the same batch.
The disks are likely to have been shipped at the same time, thus they are likely to have suffered from the same transportation damage.
As installed both disks are attached to the same power supply, making them vulnerable to the same power supply issues.
As installed both disks are in the same case, making them vulnerable to the same overheating events.
They will be both attached to the same card or motherboard, and driven by the same software, which may have the same bugs.
Because of the very nature of RAID1, both disks will be subjected to the same workload and very closely similar access patterns, stressing them in the same way.

Also, if the events of failure of two components are maximally statistically dependent, the probability of the joint failure of both is identical to the probability of failure of them individually. In such a case, the advantages of redundancy are negated. Strategies for the avoidance of common mode failures include keeping redundant components physically isolated.

A prime example of redundancy with isolation is a nuclear power plant

Nuclear power

Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...

. The new ABWR

Advanced Boiling Water Reactor

The Advanced Boiling Water Reactor is a Generation III boiling water reactor. The ABWR is currently offered by GE Hitachi Nuclear Energy and Toshiba...

has three divisions of Emergency Core Cooling Systems, each with its own generators and pumps and each isolated from the others. The new European Pressurized Reactor

European Pressurized Reactor

The EPR is a third generation pressurized water reactor design. It has been designed and developed mainly by Framatome , Electricité de France in France, and Siemens AG in Germany...

has two containment building

Containment building

A containment building, in its most common usage, is a steel or reinforced concrete structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radiation to a maximum pressure in the range of 60 to 200 psi...

s, one inside the other. However, even here it is not impossible for a common mode failure to occur (for example, caused by a highly-unlikely Richter 9 earthquake

Earthquake

An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

Origins and concepts

Common-cause variation

Special-cause variation

Common causes

Special causes

Importance to economics

Importance to industrial and quality management

Deming and Shewhart

Keynes

In engineering

See also