by John R. Grout



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Additions, corrections, & questions

100% inspection -- performing an inspection on every unit produced -- the inspection interval is the time required to produce one unit.


active error -- an active error has immediate consequences. Compare to latent errors.

altogether methods -- method used to insure that all parts of an operation have been completed. See fixed value method.

action-based error detection -- Sellen has determined that there are three general mechanisms for detecting errors -- action-based, outcome-based, and limiting functions. Action-based detection is an immediate detection of the error based on a perception that the action was different than what was intended, or planned. This detection mechanisms does not rely on cues from the environment, but is rather an internal function.

action slips -- an error where the intent was correct but action did not occur as intended.

action step methods -- setting function discussed by Monden. It requires workers to perform a step that is not part of the operations on the product, like insuring the worker inserts their hand in the correct parts bin. See also motion-step method.

activation-trigger-schemata system -- actions which are commonly performed over time will become established as unconscious routines called schemas (or schemata, both are plural of schema). When the conscious mind intends to perform some action a group of relevant schemas are activated. Activated schema stand ready, waiting for specific triggers or cues from the environment in order to actually execute the schema. Errors result from unintentional activation of inappropriate schemas, loss of activation through interruption or forgotten intention, or faulty triggering of active schemas -- either false triggering by a similar cue, or failure to trigger. The more highly active a schema is, the easier it is to trigger it with similar cues Interruptions and interference by the conscious mind also play a major role in faulty triggering. (Stewart)

adjustment error -- number 8 of Nikkan Kogyo Shimbun (Factory Magazine) top 10 sources of defects

alarms -- one of Shingo's five basic classes of poka yoke devices. Used to cue workers to take action when an error or defect has occurred. See warning methods

associative activation errors -- a type of slip identified by Norman that is caused by an internal association of two activities. For example, calling children by their siblings' name.

ATS -- see activation-trigger-schemata

attribute control charts -- a control chart used to plot data that is not based on actual numerical measurements. Attribute control charts include proportion defective (p) charts, number defective (np) charts, count of defects (c) charts, average number of defects (u) charts.

automation-induced complacency -- the inattention that results from a human monitoring reliable automated equipment. As the perceived need for active intervention declines, the mind wanders. The ability to assess the state of the process and appropriately intervene is reduced.

autonomation -- the automatic control of defects. It includes the practice of stopping machines when some deviation or variance is detected. The detection and stoppage can be effected by either a worker or a machine. Detection schemes include contact, altogether, and action step methods. Mistake-proofing as a quality control technique is a subset of autonomation. Autonomation can also be employed for other, non-quality, purposes.


baka-yoke -- precursor to poka-yoke. It is translated from Japanese as fool-proofing. Shingo discontinued its use when a worker took exception to the implication that she was a fool.


capture errors -- a type of slip identified by Norman. A frequently done activity is executed instead of the intended one. A change from the intended activity to the frequent activity may occur in a portion that both activities have in common . For example, failing to stop at a store that is on the way home from work. Most of the route to the store is identical to the trip home. The trip home occurs more frequently. A capture error has occurred when you find yourself at home having meant to go to the store.

checklists -- one of Shingo's five basic classes of poka yoke devices

cognitive black box. -- a mechanism within the human mind that selects the appropriate rule or routine, or directs that conscious deliberative effort be applied. Relevant information about the problem or task, along with other environmental factors are inputs to a person's cognitive black box. The output is some action or decision.

cognitive control level -- the selection of cognitive control method from the skill-, rule-, and knowledge-based cognitive control. Knowledge-based tasks involves a higher level of cognitive control than does skill-based.

cognitive demons -- term for schemata used by Reason and Mycielska. In part, this is due to the way they are summoned by the conscious mind to perform acts without its intervention. The primary reason, however, is because they are quite unruly, often activating without being summoned, wandering away to become inactive if not immediately triggered, being cued into action by the wrong trigger, or missing their cue, leaving action in the hands of another demon. It is this unruly nature which is the source of action slips. (Stewart)

cognitive engineering -- Rasmussen (1988) coined the term meaning designing products and processes using the error control strategies proposed by psychologists and systems designers.

complexity -- no change from common usage. Complexity in assembly is measured by the difference between theoretical minimum operation times and projected actual operation times.

contact methods -- "methods in which sensing devices detect abnormalities in product shape or dimension by whether or not contact is made" (Shingo). These methods are usually used to check size, shape, orientation, presence/absence, or appearance. Also called physical poka-yokes.

control charts -- the chief tool of SPC. A time series plot of process statistics used to monitor the stationarity of the process and detect when changes to the statistic appear to have occurred.

control limits -- a statistical concept that indicates regions where a random variable's values are considered to be statistically unlikely (unusual) for a given process. Often these limits are 3 standard deviations above and below the mean. When an observation is further from the mean than the control limit, one infers that a special cause of variation exists and that remedial action to control the process is appropriate.

control methods -- devices are designed to stop production until any improper conditions are remedied.

counters -- one of Shingo's five basic classes of poka yoke devices

counting poka-yokes -- mistake-proofing devices that monitor or check the number of repetitions of an action and warn or control the process when the correct number of actions have not been completed. Synonym for fixed-value methods

cultural constraint --a mechanism for putting knowledge in the world by adhering to a known convention. The use of the color red to indicate caution or warning and the color green to indicate proper or acceptable status


data-driven errors -- a type of slip identified by Norman where an action is triggered by the arrival of sensory data. You get out your credit card before placing a catalog order by phone. You accidentally start dialing your credit card number instead of the phone number.

decouplers -- inventory holding locations in automated manufacturing cells, usually hold only a few work pieces at a time. Decouplers can be used for automated inspection. Time that would otherwise be idle is used to perform quality checks.

defect -- process outputs that do not function properly or do not meet accepted standards or expectations. Often defects are those items that fall outside the tolerance or specification limits.

description errors -- a type of Slip identified by Norman where the intended action has much in common with others that are possible. The mind's description of the intended action is not precise enough. This slip usually is manifest as performing the right task on the wrong object like opening the cupboard door and looking for the carton of milk.

design-for-manufacturability and assembly -- a set of tools or practices intended to help engineers or designers create product designs that are more economically fabricated and assembled. Hinckley shows that use of design-for-manufacturability and assembly can be linked to improved quality. See also producibility triad.

detecting errors -- Sellen has determined that there are three general mechanisms for detecting errors -- action-based, outcome-based, and limiting functions. Reason (1990) provides a fourth category, not mentioned by Sellen, detection by others.

detection by others -- Reason (1990) provides a fourth category, not mentioned by Sellen-- detection by others. This corresponds to Shingo's successive-checks.

deterministic manufacturing -- Barkman's approach to process control. The usage of deterministic here is not the opposite of stochastic. Rather deterministic is the adjective meaning "of determinism," or of "the doctrine that everything is entirely determined by a sequence of causes" (Webster's Dictionary). An approach to manufacturing based on the belief that "there is a specific reason or set of reasons for the condition of the quality of the output of a process."

DFMA -- see design-for-manufacturability and assembly

double-capture -- one of six common mechanisms for producing slips identified by Reason. The mechanism where attention is captured by some distraction and some triggering cue is missed, and the activity is captured by the most active schema (usually the most commonly used alternative leading away from the point where the cue was overlooked). See capture errors.


economic design of control chart -- the optimization problem used to determine the sample size, sample interval, and control limits that minimize the total relevant quality costs.

environmental factors -- aspects of job design that influence the frequency of human error. These factors can include ergonomic factors, lighting, temperature, work pace, fatigue, etc.

equipment not set up properly -- number 9 of Nikkan Kogyo Shimbun (Factory Magazine) top 10 sources of defects

ergonomics --"The systematic application of knowledge about the pyschological, physical, and social attributes of human beings in the design and use of all things which affect a person's working conditions: equipment and machinery, the work environment and layout, the job itself, training and the organization of work." (Humansystems Inc).

error due to misunderstanding -- one of Nikkan Kogyo Shimbun (Factory Magazine) 10 types of errors. When the workers jump to wrong conclusion because they are not familiar with the situation.

error setting up workpieces -- number 3 of Nikkan Kogyo Shimbun (Factory Magazine) top 10 sources of defects

error tolerant -- a system where the results of committing errors are relatively benign.

error -- the failure of planned actions to achieve their desired goal. The execution of a prohibited action, the failure to correctly perform a required action or the misinterpretation of information essential to the correct execution of an action. These are often categorized as slips and mistakes depending on whether the intent was correct. See Slips and mistakes.

errors due to lack of standards -- one of Nikkan Kogyo Shimbun (Factory Magazine) 10 types of errors. Errors that occur because no instructions or standards exist.

errors due to slowness -- one of Nikkan Kogyo Shimbun (Factory Magazine) 10 types of errors. Actions are slowed down due to delays in judgment which result in error.

errors in identification -- one of Nikkan Kogyo Shimbun (Factory Magazine) 10 types of errors. Situation is misjudged either because assessment was performed too quickly or from too far away

errors made by amateurs -- one of Nikkan Kogyo Shimbun (Factory Magazine) 10 types of errors. Errors that result from lack of experience.


fail-safing -- 1. synonymous with mistake-proofing or 2. "a system designed so that if anything goes wrong, it immediately becomes impossible for the negative impact to continue" (Royal).

failure mode and effects analysis -- a systematic review of potential product or process problems and their effect on the product or process. Failure mode and effects analysis is normal done on a form which also facilitates the prioritization and management of remedial actions to reduce the occurrence of failure modes or minimize their effects.

fault tree analysis -- a graphical management tool for describing the cause and effect relationships that result in major failures. The fault or major failure being analyzed is identified as the "top event." All of the possible causes of the top event are identified in a tree using "or" nodes for independent causes and "and" nodes for multiple causes that must exist concurrently for a failure to occur.

fixed value methods -- inspections used to determine if a specified number of motions occur a predetermined number of times.

fixtures -- a device that holds the workpiece securely in the correct position relative to the tool in a manufacturing process but does not guide the tool (as opposed to a jig).

FMEA -- see failure mode effect analysis

fool proofing -- a pejorative term occasionally used as a synonym for mistake-proofing. See also baka-yoke.

forcing functions -- term used by Norman meaning a physical constraints that insures correct actions occur or that restrict actions so that failure at one stage prevents the next process step from occurring.

forgetfulness -- one of Nikkan Kogyo Shimbun (Factory Magazine) error types. Errors that result from not concentrating and forgetting things.


gauge -- a measurement device for determining the dimension of a workpiece

GEMS -- see generic error modeling system

generic error modeling system -- Reason has built on the SRK framework, collecting different mechanisms from the literature (including the ATS framework) and assigning them to the appropriate cognitive levels. He refers to this enhanced framework as a generic error modeling system (GEMS). GEMS is a more general description of the cognitive black box, which can be used to address the mechanisms of both slips and mistakes. (Stewart)

go/no-go gauges -- a gauge that provides categorical data about whether one or more dimension of a workpiece is within specification limits.

grouping poka-yokes -- devices designed to determine when objects have been omitted or when extra objects have been included. See fixed-value methods.

guide pins -- one of Shingo's five basic classes of poka yoke devices


HRA -- see human reliability assessment

human factors -- the characteristics of human beings that are applicable to the design of systems and devices of all kinds. It furthers serious consideration of knowledge about the assignment of appropriate functions for humans and machines, whether people serve as operators, maintainers, or users in the system. And, it advocates systematic use of such knowledge to achieve compatibility in the design of interactive systems of people, machines, and environments to ensure their effectiveness, safety, and ease of performance.

human reliability assessment -- similar to the engineering technique of probabilistic risk assessment. A given design is evaluated to determine some probability of human failure. The final approach focuses on employee training. Operators can be taught about error theory allowing them to identify circumstances where they are likely to be error prone. They can also be trained in better diagnostic procedures to increase efficiency and accuracy of trouble-shooting activities. And, simulators can be employed to allow operators to learn from errors made "off-line." (Stewart)


inadvertent errors -- one of Nikkan Kogyo Shimbun (Factory Magazine) error types. An absentminded error that occurs without the individual knowing how it happened.

information poka-yokes -- poka yokes designed to ensure that information required in the process is available at the correct time and place, and that it stands out against a noisy background. (Chase and Stewart)

informative inspection -- inspection uses data gathered from production to understand and control the process in the future. Statistical process control is one type of informative inspection. Shingo introduced two additional types of informative inspection: successive checks and self-checks.

inspection cost -- those costs, both fixed (invariant in sample size) and variable(linearly related to sample size), arising directly from taking a sample and calculating any appropriate statistics.

inspection interval -- the duration between inspections expressed in units of time or quantity produced.

intention -- a determination to act in a certain way (Webster)

intentional errors -- one of Nikkan Kogyo Shimbun (Factory Magazine) error types. wantonly deviating from proper or acceptable actions, processes or procedures. Worse than a "willful error" because it implies malicious intent, crimes and sabotage being typical.

interference errors -- one of six common mechanisms for producing slips identified by Reason. occur when multiple active schemas become confused.

interlock -- a device or set of devices that insures the proper sequence of actions occur. Most cars have interlocks to insure certain aspects of proper operation. The key must be inserted before the steering wheel will turn or the transmission shifted into gear. The key cannot be removed until the car is put in park.


jidoka -- A Japanese term that can be translated two ways: 1) automation "improvements in machinery that serve to reduce the number of workers" and 2) autonomation, see definition above. (Monden)

jig -- a device that holds the workpiece securely in the correct positions and has the capability of guiding the tool during a manufacturing operation.

judgment inspection -- inspects process output and separates the defects from the acceptable products. Shingo points out that judgment inspection is not an effective quality management approach.


knowledge-based cognitive control -- if there are no rules which apply, the person switches to pure logical deduction based on theoretical knowledge of how the system behaves. This is called knowledge-based cognitive control, and it requires substantial cognitive effort.

knowledge in the head -- information that must be retain in memory in order to correctly complete a task. Tasks requiring more knowledge in the head are more error-prone.

knowledge in the world -- information that is embedded, encoded or implicitly resides in the components of a task. The components and steps of a task communicate all the information needed to perform the task correctly.


latent errors -- errors that are made for which the consequences do not occur immediately. Latent errors, once made, sit idle in the system until conditions allow them to be manifest. These errors tend to be hard to predict or prevent. Compare to active errors.

Level 1 device -- A device that prevents a mistake or eliminates the error

Level 2 device -- A device that detects a mistake or error after it occurs, but before it turns into a defect (see source inspection)

Level 3 device -- A device that detects a defect (see self-check)

limit switches -- one of Shingo's five basic classes of poka yoke devices. A small electronic switch that closes when it is depressed. They are typically used to insure that items are in the correct position.

limiting functions error detection -- Sellen has determined that there are three general mechanisms for detecting errors -- action-based, outcome-based, and limiting functions. The last mechanisms, limiting functions, cause error detection by preventing further action in the presence of an error. Limiting functions are the same as Shingo's control poka-yokes (1985). A lengthy discussion of limiting functions can be found in Norman (1988), although he refers to them as "forcing functions".

lockins -- one of Norman's forcing functions that avoids premature termination of a process

lockouts -- one of Norman's forcing functions that prevents an event from occurring

logical constraint -- putting knowledge in the world using designs that are based on making sense of relationships

loss-of-activation errors -- a type of slip identified by Norman where one action is executed but the goal of the action or a subsequent action is forgotten. For example, you intentionally walk to the garage to get a tool but by the time you get there you have forgotten which tool you needed.


measurement control charts -- control charts that track data that is quantitative, as opposed to attribute data. Measurement control charts include mean (x-bar) charts, range (R) charts, standard deviation (s) charts.

misoperation -- number 7 of Nikkan Kogyo Shimbun (Factory Magazine) top 10 sources of defects

missing parts -- number 4 of Nikkan Kogyo Shimbun (Factory Magazine) top 10 sources of defects

mistake-proofing -- improving processes or designs to prevent mistakes from being made or to make the mistake obvious at a glance. Error-proofing, ZQC, or poka-yoke can be used as synonyms.

mistake detection -- involves discovering the occurrence of defects. This concept corresponds to Shingo's self-checks and successive checks. One of four classes of mistake-proofing proposed by Tsuda.

mistake prevention -- the elimination of the possiblility of making a mistake. Shingo's source inspection is mistake prevention. One of four classes of mistake-proofing proposed by Tsuda.

mistake prevention in the work environment -- avoiding ambiguity in the environment. This would include process simplification, good housekeeping, and quality and safety training. One of four classes of mistake-proofing proposed by Tsuda.

mistakes -- 1. (strict definition) conscious deliberation that leads to selecting the wrong intention. 2. (common definition) synonym for error. For example, the term mistake-proofing uses the common definition since mistake-proofing is used more to prevent slips than mistakes (using strict definition)

mistimed checks by the conscious mind -- a common mechanisms for producing slips identified by Reason. progress checks made mid-schema interrupts the schema, and if the amount of progress that has been made is not clear, it may be resumed later in the process (resulting in an omission) or earlier in the process (resulting in a repetition). (Stewart)

mode errors -- a types of slip identified by Norman that occurs because the same controls are used differently in different modes of operation. The error occurs when an action that would be correct in one mode is executed while unwittingly in another mode. For example, making corrections in electronic text by typing over existing text when you intended for the existing text to be pushed out to make room for your typing.

motion step methods -- when abnormalities are detected by checking that standard predetermined motions are executed correctly. See also sequencing poka-yokes.


nonconformity -- some aspect of a product that does not conform to its specifications


omissions following interruptions -- one of six common mechanisms for producing slips identified by Reason. result when the schema is interrupted by an outside influence, and either fails to be completed, or resumes action sometime later in the process.

omitted processing -- number 1 of Nikkan Kogyo Shimbun (Factory Magazine) top 10 sources of defects

outcome-based error detection -- Sellen has determined that there are three general mechanisms for detecting errors -- action-based, outcome-based, and limiting functions. Outcome-based detection relies on a comparison of the manifested outcome with the expected outcome or with familiar error patterns. Aiding the search for familiar error patterns seems to be the primary strategy behind Shingo's self-check poka-yoke devices.


perceptual confusion -- one of six common mechanisms for producing slips identified by Reason. It occurs when a schema acts on a similar, but inappropriate object. Objects which are similar in appearance, location, or function may be used erroneously by the schema. Examples include putting tea in the coffee machine. (Stewart)

physical constraint -- putting knowledge in the world using designs that rely on the shape and size of objects to control their relationship

physical poka-yokes --synonym for Shingo's contact methods

poka-yoke -- poka is Japanese for "inadvertent mistake". Yokeru is Japanese for "to avoid." It is often used as a synonym for ZQC, error-proofing, or mistake-proofing.

poka-yoke devices -- mechanisms that either prevent a mistake from being made or make the mistake obvious at a glance.

poka yokay -- alternate transliteration of poka-yoke used by Bandyopadhyay (1993)

preemptive process inspection -- synonym for source inspection. Source inspections are preemptive because they discover errors before defects occur or prevent errors from happening. They are also inspections of the process not the inspection of products.

preventing the influence of mistakes -- allowing the mistake to occur but limiting the consequences. One of four classes of mistake-proofing proposed by Tsuda. Tsuda uses antilock braking systems as an example. The error in using the brakes is pressing too hard on the brake pedal causing a skid. Antilock brakes allow the brake to be pressed hard without initiating a skid.

process capability -- an assessment of the ability of a process to consistently produce outputs that conform to specifications

processing errors -- number 2 of Nikkan Kogyo Shimbun (Factory Magazine) top 10 sources of defects

processing wrong workpiece -- number 6 of Nikkan Kogyo Shimbun (Factory Magazine) top 10 sources of defects

producibility triad -- three classes of defect sources: variation, mistakes, and complexity. statistical process control deals with variation. use of poka yoke to avoid and eliminate mistakes. the use of design-for-assembly methodologies for reducing complexity



reduced intentionality -- one of six common mechanisms for producing slips identified by Reason. refers to the mechanism where the schema is activated, but some delay intervenes before it is triggered. By the time the triggering cue occurs, the schema has lost activation, and the intention has been forgotten.

regulatory function -- a warning, notification, or cue to the worker that a process parameter or a product attribute is incorrect. See warning methods, and control methods.

rule-based cognitive control -- when faced with a more unusual situation or problem, people will look for familiar patterns for which they have stored rules about how to respond. This is called rule-based cognitive control, and generally requires little cognitive effort. Readers familiar with expert systems will perceive striking similarities to rule-based cognitive control. This is due to a shared pedigree. (Stewart)


schema -- knowledge structures that contain the general rules and information necessary for interpreting situations and for guiding behavior. Schema are sometimes thought of as "subroutines" that the human mind uses to control routine behavior, on "autopilot." Skill-based cognitive control is accomplished using schema.

schemata -- plural of schema

self-checks -- each operation assesses the quality of their own work using 100% inspection. Because they check every unit produced, they may be able to recognize what circumstances changed that caused the last unit to be defective. This insight is used to prevent further defects. The difference between successive checks and self-checks is who performs the inspection. For most situations, self-checks are preferred to successive checks.

semantic constraint -- putting knowledge in the world using designs that rely on clues from the meaning of the situation

sensors -- a device that responds to a particular stimulus and transmits a resulting impulse (Webster).

sequencing poka-yokes -- "indicate, discourage, or prevent deviations from the order of steps in a process" (Chase and Stewart). See also motion-step methods.

setting function -- the method used to detect errors or incorrect process parameters or product attributes. The setting function is linked to a regulatory function. See also contact methods, fixed-value methods, motion-step methods.

skill-based cognitive control -- common activities in routine situations are handled at skill-based level of control, which operates with little conscious intervention. This is the domain of the action slip. See Schema.

skill-rule-knowledge framework -- the premise of Rasmussen's framework is that people switch among different levels of cognitive control when faced with different situations. Different tasks require different amounts of attention, training, and practice. Familiar tasks require less attention and are performed using skill-base cognitive control. Less familiar tasks utilize rule-based or knowledge-based cognitive control.

slip -- the result from skill-based behavior, when subconscious actions that are intended to satisfy a goal fail to result in the intended outcome.

source inspection -- uses poka-yoke devices to detect improper operating conditions prior to actual production. Source inspection is preferred to informative inspection because remedial action can be taken before a defect is produced. See preemptive process inspection

SPC -- see statistical process control

SQC -- see statistical quality control

SRK -- see skill-rule-knowledge framework

statistical process control -- an informative inspection technique that uses sampling and control charts to determine if production processes are stationary and to indicate when shifts or changes are likely to have occurred in the process.

statistical quality control -- term used by Shingo. Based on his usage, it includes most common statistical techniques for quality control: sampling, design of experiments and statistical process control. Shingo often uses statistical quality control as a synonym for statistical process control.

stored action routines -- see schema

successive checks -- an operation performs 100% inspection of the work of the prior operation.

surprise errors -- one of Nikkan Kogyo Shimbun (Factory Magazine) 10 types of errors. Error occurs when equipment runs differently than expected.


tools and jigs improperly prepared -- number 10 of Nikkan Kogyo Shimbun (Factory Magazine) top 10 sources of defects

type I error -- in statistical process control, incorrectly inferring the process is out of control when the process is actually in control. In hypothesis testing, incorrectly rejecting the null hypothesis.

type II error -- in statistical process control, incorrectly inferring the process is in control when the process is actually out of control. In hypothesis testing, incorrectly failing to reject the null hypothesis.



variance -- a statistical measure of variation in a set of data. The average squared deviation of data points from the mean.

variation -- the act or process of varying, divergence in a characteristic from an average or norm. (Webster)

violation -- consciously selecting an incorrect intention that is executed as planned (usually a choice to not follow procedures and use a "work-around" instead).


warning methods -- the use of mistake-proofing devices to call attention to improper operating conditions but not to stop production.

willful errors -- one of Nikkan Kogyo Shimbun (Factory Magazine) 10 types of errors. Worker or user decides to ignore the rules. See violation.

work-around -- an informal or unapproved procedure that workers use instead of formal documented procedures because the informal procedures are perceived to be less difficult or time consuming and appear to achieve the same outcome as the formal procedure.

wrong parts -- number 5 of Nikkan Kogyo Shimbun (Factory Magazine) top 10 sources of defects




zero quality control -- a system based on source inspection to detect defects at their source, 100% inspection using inexpensive sensing devices, and immediate feedback and corrective action. Sometimes used as a synonym for mistake-proofing, error-proofing or poka-yoke (in a broad sense)

ZQC -- see zero quality control

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Glossary of Error-Proofing Terms
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