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Is there such a thing as a single root cause?

On a Linked In site operated by the official group for the W. Edwards Deming Institute, a member posed the question, "Is there such a thing as a single root cause?" Several other members posted responses. As is too often the case, some of the responses took the conversation into an unnecessary level of complexity. For example, one respondent wrote,

"...Often I think you may find that the results are not very robust and this time we caught the failure because of u = 11, x = 3, y = 4 and z = 1. But those with knowledge working on the process can tell the results are not reliable unless x = 5 or 6. And if z is under 3 things are likely to go wrong. And if u is above 8 and x is below 5 and y is below 5 things are in trouble.

"To me this often amounts to designing systems to be robust and able to perform with the variation that is likely to happen. And for those areas where the system can't be made robust for some variation then design things so that variation doesn't happen to the system (mistake proofing measures for example)..."

[Am I the only one who's getting a little tired of overly complex responses to relatively simple questions? For more on this topic, see my blog, "Beware Adding Unnecessary Complexity: An On-Line Debate with Dr. Mikel Harry." It was posted on this web site on March 15, 2014.]

I decided to pitch in and try to answer the original question as follows:

There can be a single "root" cause in the case of an out-of-control process. In such a case, one can identify and address the special cause (singular) of the event. On occasion, however, the special cause of the significant variation may be the result of an interaction. For example, a new material may work well with several door designs, but doesn't work with the pressure setting for another door. The new material alone didn't cause the significantly high failure rate, nor did the pressure setting. The interaction between the two variables would be the special cause (singular).

On the other hand, there will be no single root cause in the case of common cause variation. Random, systemic variation is the result of common causes (plural) and interactions between and among common causes (plural) of variation. The futile search for a root cause (singular) in such cases is the commission of Deming's Mistake 1, tampering.