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| articles:whac-a-mole [2019/05/29 10:21] – [Corrective Action... and "Whac-A-Mole"] rrandall | articles:whac-a-mole [2020/12/04 13:20] – [Common Cause vs Special Cause Variation] rrandall |
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| | **Special Cause** | unusual / abnormal,\\ not previously observed,\\ non-quantifiable variation | | | **Special Cause** | unusual / abnormal,\\ not previously observed,\\ non-quantifiable variation | |
| </WRAP> | </WRAP> |
| 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. | 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. |
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| 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. | 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. |
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| 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. | 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. |
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| The [[https://support.minitab.com/en-us/minitab/18/help-and-how-to/quality-and-process-improvement/control-charts/supporting-topics/basics/using-control-charts-to-detect-variation-in-a-process/|Minitab® 18 Support web site]] offers the following table as "**Examples of common-cause and special-cause variation**". | The [[https://support.minitab.com/en-us/minitab/18/help-and-how-to/quality-and-process-improvement/control-charts/supporting-topics/basics/using-control-charts-to-detect-variation-in-a-process/|Minitab® 18 Support web site]] offers the following table as "**Examples of common-cause and special-cause variation**". |
| | Injection molding of plastic toys | Slight variations in the plastic from a supplier result in minor variations in product strength from batch to batch. | Changing to a less reliable plastic supplier leads to an immediate shift in the strength and consistency of your final product. | | | Injection molding of plastic toys | Slight variations in the plastic from a supplier result in minor variations in product strength from batch to batch. | Changing to a less reliable plastic supplier leads to an immediate shift in the strength and consistency of your final product. | |
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| 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 the 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 through 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. | 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 the 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. |
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