Whac-A-Mole Arcade Game

Corrective Action "Whac-A-Mole"

While a “Corrective Action” has traditionally been considered an essential component for any quality management system, surprisingly few quality professionals have a good understanding of it. This is most often because so many quality professionals lack a basic understanding of “Common Cause” vs “Special Cause” (aka “Assignable Cause”) variation in a process.

These concepts were first described by Walter A. Shewhart in his 1931 book, “Economic control of quality of manufactured product”. And promoted by the Western Electric Company in its 1956 book, “Introduction to Statistical Quality Control handbook” (1 ed.). Later, these concepts were popularized by W. Edwards Deming in his 1982 book, “Out of the Crisis: Quality, Productivity and Competitive Position”. Unfortunately, there are still many quality professionals who remain oblivious to these concepts!

Common Cause vs Special Cause Variation

Imagine that you frequent a particular restaurant. The service and food are typically great, but occasionally you find that the silverware (rolled in a napkin) is missing a piece… or has two of the same piece while missing the third piece (e.g., two forks and no knife). Whenever this happens, you simply inform the server, who immediately provides you another napkin neatly rolled around all of the required silverware. The “nonconforming” condition is quickly and easily corrected.

Common Cause usual / normal
quantifiable, random variation
Special Cause unusual / abnormal,
not previously observed,
non-quantifiable variation

The staff is well trained and rarely make these mistakes. If one were to initiate a “corrective action” to address this situation, it would quickly prove futile because there is NO “assignable” root cause to be eliminated. This type of error is a normal, random (common cause) variation in the restaurant's process due to the volume of silverware that is washed, sorted, and manually rolled into napkins by the staff.

However, one night you arrive and order dinner only to find that the food takes much longer than normal to arrive at your table, has been over-cooked, and is cold. You complain to the server who apologizes explaining that their chef is unexpectedly absent tonight due to an illness. And that this was terrible timing because his Assistant chef is traveling on vacation. Consequently, a replacement chef, who is unfamiliar with both their kitchen and menu, had to be brought in to fill this temporary need.

Unlike the silverware issue, this is an unusual/abnormal, situation that has not previously occurred. Therefore, this would be a “special cause” variation in their process with an “assignable” root cause. Special causes produce systematic effects/errors in a process.

The Minitab® 18 Support web site offers “Examples of common-cause and special-cause variation” in a table (provided below with a few minor improvements).

Process Common Cause of Variation Special Cause of Variation
Baking a loaf of bread The control loop in the oven's thermostat allows the temperature to drift up and down slightly. Changing the oven's temperature or repeatedly opening the oven door during baking can cause the temperature to fluctuate needlessly.
Recording customer contact information An experienced operator makes an occasional error. An untrained operator new to the job makes numerous data-entry errors.
Injection molding of plastic toys Slight variations in the plastic resin from a supplier result in minor variations in product strength from batch to batch. Changing to a less reliable plastic resin supplier leads to an immediate shift in the strength and consistency of your final product.

As shown in the above examples, a control chart isn't always needed to differentiate between a “Common Cause” and “Special Cause” variation. However, because far too many quality professionals fail to differentiate between “Common Cause” and “Special Cause” variations in a process, a very large number of nonconformities from “Common Cause” variations are incorrectly addressed through a corrective action process. This leads to a cycle resembling the 1976 arcade game, “Whac-A-Mole” (where moles pop up from their holes at random, and the player earns points by forcing them back into their hole by hitting them directly on the head with a mallet). In the end, nothing is accomplished… other than the player feeling a false sense of accomplishment reflected by their score. In this case, the quality team “feels” good (a false sense of accomplishment) about the apparent (short term) success of each corrective action.

Reactive vs. Proactive Process Improvement

Reactive Process Improvement

In order to properly implement a “corrective action”, there must be an assignable “root cause”. And in order for there to be an assignable “root cause”, the nonconforming condition must be the result of a “special cause” variation (i.e., an “unnatural pattern” in the process, which is unusual, not previously observed, non-quantifiable variation in the process).

Consequently, because “special causes” produce systematic effects/errors in a process, properly implemented “corrective actions” are reactive process improvements… refining an existing process.

Proactive Process Improvement

Because “common cause” variations are inherent to the existing process, these variations can only be reduced or eliminated by significantly “modifying” or “re-designing” the existing process. The most effective process improvement activities are realized by applying the Lean Six Sigma concepts & methodologies.


Ironically, a large portion of the traditional quality profession has been obsessed with achieving “Zero Defects” (promoted by Philip B. Crosby). And whether ignoring or failing to understand the difference between “Common Cause” and “Special Cause” variation, the vast majority of nonconformities are addressed as “Special Cause” variations without regardless of their true cause!

Upon understanding “Common Cause” and “Special Cause” variation in a process, one soon realizes that these are both associated with “risk management”. And the best tool used for addressing quality-related risk is the "Failure Mode and Effects Analysis" (FMEA)… or FMECA (“Failure Mode, Effects and Criticality Analysis”).

IF a nonconformity is due to a “Common Cause” variation, then “controls” can be put in place to “mitigate” the risk of the nonconformity recurring. Alternatively, IF a nonconformity is due to a “Special Cause” variation, then the process can be changed (e.g., through the implementation of “controls”) to “eliminate” the cause (or risk) of the nonconformity recurring.

Upon completing a PFMEA (“Process Failure Mode and Effects Analysis”), one quickly realizes just how few “true” corrective actions are actually possible. And that by shifting the mindset to “risk management”, rather than being obsessed with “risk elimination” (aka “Corrective Action”), significant quality improvements can finally be realized.