Assignable Causes Of Variation

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Common causes of variation often lie hidden within the system, and are sometimes assumed to be unavoidable.

Yet it is very possible, and often very rewarding, to improve processes and reduce common cause variation.

Those six elements – 5 Ms and on P (or 6Ms) influence variation in all processes – manufacturing or not.

Walter Shewhart developed the idea of the control chart in the 1920’s to help decide when the output of a process was part of “a stable system of chance causes”, or whether there was an “assignable cause.“Common cause variation are the usual, historical, quantifiable variation in a system.

Improving a stable process is somewhat more difficult than improving an unstable process because, by definition, a stable process has no special causes of variation that jump out at you, asking to be investigated.

Instead, you are faced with the task of looking at all data about the process, not simply what made one point different from the others.Common causes are the normal, expected variances that occur.Common causes are predictable and are not considered unusual.When a work process has only common causes of variation and no special causes, that process is "in control." This means that it is stable, consistent, and predictable.It might be predictably good or predictably bad, or it might be a very regular mix of good and bad results. Just because a process is stable, or in statistical control, does not mean that its results are satisfactory.On a control chart, common causes of variance would be indicated by the random points within the control limits.Common causes of variance are also known as “random” causes.Designed to help those that are preparing to take the PMP or CAPM Certification Exam, each post within this series presents a comparison of common concepts that appear on the PMP and CAPM exams.These quality definitions refer to variances or variations in systems, processing, or tested results.(A) Decrease the target fill volume only (B) Decrease the target fill variation only (C) First decrease the target fill volume, then decrease the target fill variation (D) First decrease the target fill variation, then decrease the target fill volume.Common Causes vs Special Causes of Variance is the 5th post in our PMP Concepts Learning Series.


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