Project Quality Management: Common Cause vs Special Cause for PMP Exam

There are always variances for the results from a process over time. The results are to be plotted on a control chart. The closer the results from each other, the smaller the variance (a variance of 0 indicates that all the results are identical — however this cannot be achieved in reality). The variance may be caused by a number of factors, which can be broadly divided into two category: Common Cause and Special Cause. The type of variance cause can be analyzed by the pattern shown on the control chart.

Common Cause vs Special Cause

• Common Cause: the normal variances for every process which in random in nature — inherit in the process (life is not perfect)
  • Common cause is normal and cannot be elimiated
  • On a control chart, common cause variations would have the pattern of:
    • all points within the control limits
    • points distributed randomly on both sides of the average value
  • Common cause variance is also known as random cause — i.e. there is not a special reason for the variation
  • The process in question is considered as stable
• Special Cause: causes that are NOT inherent in the process
  • Special cause indicates that there may be some sort of defects in the process and the cause of the variance needs to be dug out
  • Special cause can usually be resolved with adjustments to the processes, components or methods
  • On a control chart, special cause variations would have the pattern of either:
    • a point or more beyond the control limits
    • some trends of the points (e.g. more than 5 consecutive points on one side of the average value
  • Special cause is also know as assignable cause — that can be attributed to some special reasons

Illustrated Example

Let’s again take the project of PMP Exam preparation as an example.

When working on some mock exam papers, your score for 5 different mock question sets were:

• 72%
• 70%
• 68%
• 69%
• 73%

All these scores were within the control limits of 3 standard deviations with the mean of 70.4% (according to the experience of many Aspirants, a mean of 70.4% would mean that one stands a high chance to pass the exam in first try). The variations in the mock exam results was due to common causes.

However, after attempting the 6th mock exam question, you got 29% (which is well below the control limits). The deviation must be due to special causes and you needed to dig out the underlying reasons. Below are some possible causes:

• this mock exam paper is extremely difficult
• the questions ask you about knowledge that is outside the exam syllabus
• the quality of the mock exam paper is questionable (maybe many of answers to the questions were wrong or even the questions were wrong)
• you are not familiar with the knowledge areas covered specially in this mock exam
• in case you are taking the exam in paper, you have mismatched the question number and the answer number
• ……

There might be lots of different explanations the unexpectedly low marks you got from this exam. You would need to try finding out which one (or more) cause from the above list is the “special causes” so that you could take actions to make things right — and to move closer to your goal of passing the real PMP exam.

Summary

Aspirants would just need to remember that common cause is “common” and that is not considered a defect while special cause is caused by a “special” reason (defect) which must be resolved in order for the process to run probably. The use of control charts can bring the matters into light and help the project manager to differentiate between Common Cause and Special Cause variance.

Additional FREE PMP resources: 47+ Commonly Confused Term Pairs with detailed explanations. If you found this article useful, you may wish to reference other Commonly Confused Term articles.