Common-Causes and Special-Causes are the two distinct origins of variation in a process, as defined in statistical thinking and methods. Common-Causes are usually, historical, quantifiable variation in a system, while Special-Causes are unusual, not previously observed, non-quantifiable variation. 

Origins and Concepts

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. Special-Cause Variation is fundamentally unpredictable in frequency of occurrence or in severity.

DEFINITIONS
Common-Cause Variation is characterised by:
*  Phenomena constantly active within the system
*  Variation predictable probabilistically
*  Irregular Variation within an historical experience base
*  Lack of significance in individual high or low values
The outcomes of a perfectly balanced roulette wheel are a good example of Common-Cause Variation, which is the noise within the system.

Originally the term Chance-Cause was used and morphed in to the term Common-Cause. (Natural Pattern). A process that features only Common-Cause Variation is in Statistical Control. This term is deprecated by some modern statisticians who prefer the phrase Stable and Predictable.

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
*  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.

EXAMPLES

COMMON CAUSES
*  Inappropriate procedures
*  Poor design
*  Poor maintenance of machines
*  Lack of clearly defined standing operating procedures
*  Poor working conditions, e.g. lighting, noise, dirt, temperature, ventilation
*  Substandard raw materials
*  Assurement error
*  Quality control 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
*  Abnormal traffic (click-fraud) on web ads[4]
*  Extremely long lab testing turnover time due to switching to a new computer system
*  Operator absent

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 for the failure.

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 interventions in processes that merely increase the level of variation and frequency of undesirable outcomes.

A Control Chart is a means of managing a Business Process in an economically efficient manner.

Redundancy (engineering) four major forms of Redundancy, these are:
*  Hardware redundancy, such as DMR and TMR
*  Information redundancy, such as Error detection and correction methods

Redundancy in information theory is the number of bits used to transmit a message minus the number of bits of actual information in the message. Informally, it is the amount of wasted or unneccary data, used to transmit the primary data. Data Compression is a way to reduce or eliminate unwanted redundancy, while Checksums are a way of adding desired redundancy for purposes of error detection when communicating over a noisy channel of limited capacity.

*  Time redundancy, including transient fault detection methods such as Alternate Logic
*  Software redundancy such as N-version programming

Function of Redundancy

The two functions of redundancy are Passive Redundancy and Active Redundancy. Both functions prevent performance decline from exceeding specification limits without human intervention using extra capacity.

Passive Redundancy uses excess capacity to reduce the impact of component failures. One common form of Passive Redundancy is the extra strength of cabling and struts used in bridges. This extra strength allows some structural components to fail without bridge collapse. The extra strength used in the design is called the margin of safety.

Eyes and ears provide working examples of passive redundancy. Vision loss in one eye does not cause blindness but depth perception is impaired. Hearing loss in one ear does not cause deafness but directionality is impaired. Performance decline is commonly associated with passive redundancy when a limited number of failures occur.

Active Redundancy eliminates performance decline by monitoring performance of individual device, and this monitoring is used in voting logic. The voting logic is linked to switching that automatically reconfigures components. Error detection and correction and the Global Positioning System (GPS) are two examples of Active Redundancy.

Electrical power distribution provides an example of Active Redundancy. Several power lines connect each generation facility with customers. Each power line include monitors that detect overload. Each power line also includes circuit breakers. The combination of power lines provides excess capacity. Circuit breakers disconnect a power line when monitors detect an overload. Power is redistributed across the remaining lines.

 Voting Logic

Voting Logic uses performance monitoring to determine how to reconfigure individual components so that operation continues without violating specification limitations of the overall system. Voting Logic often involve computers, but systems composed of items other than computers may be reconfigured using Voting Logic. Circuit breakers are an example of a form of non-computer Voting Logic.

Electrical power systems use power scheduling to reconfigure Active Redundancy. Computing systems adjust the production output of each generating facility when other generating facilities are suddenly lost. This prevents blackout conditions during major events like earthquake.

The simplest Voting Logic in computing systems involves two components: primary and alternate. They both run similar software, but the output from the alternate remains inactive during normal operation. The primary monitors itself and periodically sends an activity message to the alternate as long as everything is OK. All outputs from the primary stop, including the activity message, when the primary detects a fault. The alternate activates its output and takes over from the primary after a brief delay when the activity message ceases. Errors in Voting Logic can cause both to have all outputs active at the same time, can cause both to have all outputs inactive at the same time, or outputs can flutter on and off.

A more reliable form of Voting Logic involves an odd number of 3 devices or more. All perform identical functions and the outputs are compared by the Voting Logic. The Voting Logic establishes a majority when there is a disagreement, and the majority will act to deactivate the output from other device(s) that disagree. A single fault will not interrupt normal operation. This technique is used with avionics systems, such as those responsible for operation of the space shuttle.

 Redundancy (Information Theory)

Redundancy (Total Quality Management)

In total Quality Management, TQM, Redundancy in quality or Redundant Quality means quality which exceeds the required quality level. Tolerances may be too accurate, for example, creating unnecessarily high costs of production.

Redundant quality is sometimes incorrectly used instead of Even Quality or Constant Quality, perhaps because of the positive connotations of the term Redundancy used in connection with Safety-Critical Systems.

Redundancy (User Interfaces)

The user interface of an application software or operating system is sometimes described as redundant if the same task can be executed by several different methods. For example, a user is often able to open or save a project by navigating a menu with the mouse or keyboard, by clicking a single button with the mouse, or by entering a key stroke.

Data Redundancy

Database Normalization
*  In relational database design(RDBMS), the process of organizing data to minimize redundancy is called normalization. The goal of database normalization is to decompose relations with anomalies in order to produce smaller, well-structured relations. Normalization usually involves dividing large, badly-formed tables into smaller, well-formed tables and defining relationships between them. The objective is to isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database via the defined relationships
*  Make the data model more informative to users
Normalized tables, and the relationship between one normalized table and another, mirror real-world concepts and their interrelationships.
*  Background to normalization (Logic Options for Alarm Systems)

FUNCTIONAL DEPENDENCY
*  Trivial functional dependency
*  Full functional dependency
*  Transitive dependency
*  Multivalued dependency
*  Joint dependency
*  Superkey
*  Candidate key
*  Non-prime attribute
*  Primary key

LOGIC REDUNDANCY
*  Redundant acronym syndrome syndrome Terminology
*  Redundant computer code,  that is executed but has no effect on the output of the  program
*  Tautology (rhetoric), unnecessary repetition of meaning
*  Database normalization, the elimination of redundancy in databases
*  Removing Duplicates
*  Power Sources
*  Data Storage Mediums
*  Safety Engineering Monitoring
*  Reliability Engineering
*  Redundant Boolean Logic

Call PDQIE to Establish Redundance in Your Systems (877) PDQ-4-FIX