In what universe does a single special cause exist?

On a web site operated by the official group of the W. Edwards Deming Institute, a member posed the question, "Is there such a thing as a single root cause?"  Several other group members posted responses, and I joined the exchange.  (See my blog posted earlier on this web  site and titled, "Is there such a thing as a single root cause?")

Another group member apparently didn't agree with my post and my position that single root causes can exist in the case of special cause variation.  He posted the following response:

"I am learning a great deal from everyone's comments.  Another way of approaching the question may be to ask, 'In what universe does a single special cause exist and is that universe the one in which we currently find ourselves?'..."

I repled to the query as follows:

"In what universe does a single special cause exist and is that universe the one in which we currently find ourselves?"  Apparently, single special causes existed in Dr. Deming's universe, given that he wrote and lectured extensively about "a special cause" and "some special event."  More recently, I discovered a single special cause in the universe of one of my clients.

A significant shift in voltage performance was detected.  Upon investigation the culprit (single special cause) proved to be a component that a vendor substituted for the specified component.  Fortunately, we were able to identify the single special cause and correct the situation.  Of course, one must move beyond treatment of the special event and take appropriate action to prevent it in the future.  Such work may well involve changes to the purchasing system and/or vendor qualification process.  This strategy would be consistent with a point made by Deming in one of his lectures, "Pay attention to higher-order common cause."1

Too often, I observe Deming's Mistake 2; i.e., people getting all tied up in investigation of multiple variables or blaming the process for failures instead of responding appropriately to the statistical signal.  As Deming wrote, "One should search at once for a special cause, once it is detected, before the trail grows cold."2


1 W.E. Deming, remarks during his seminar, "Quality Productivity and Competitive Position," Chicago, IL, Sep 26-29, 1989, Sponsored by Quality Enhancement Seminars, Santa Monica, CA.

2W.E. Deming, Out of the Crisis, MIT Center for Advanced Educational Services, Cambridge, MA (1986), p. 31.

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