A process that is predictable or in a state of statistical control, but producing nonconforming product can be described as being in the Threshold State. This is one of the four possible states that a process can be in as noted in “What State Is Your Process In?“. But, what might such a process look like?
A process in the threshold state might be operating with its mean higher than the process aim,
or it might be operating with its mean lower than the process aim,
or it might be operating with a process dispersion greater than the product specification window,
or it may be operating with some combination of a shift in its mean and breadth of its dispersion.
Nevertheless, the fact that such a process is in statistical control means that it will continue to produce consistent product so long as it stays in control. This in turn means that the producer can expect to produce a consistent amount of nonconforming product hour after hour day after day until a change is made in the process or a change is made to the specifications.
It is important to say here that exhorting your workers to work harder or to “Do it right the first time” or to show them the examples of nonconforming product from a process in the threshold state will not lead to improvements. They are not the cause for the failures. The causes for the nonconforming product are systemic and must be dealt with at the system level. Focusing on the worker will only serve to demoralize and frustrate them. It may lead to tampering with the process turning a bad situation worse.
You can always share your process data with your customer to demonstrate its stability and ask for a change in the product specifications. However, if specifications cannot be changed your only recourse is to modify your process to shift it from the threshold state into the ideal state. Adjusting the process mean to match the aim is usually relatively simple. In comparison, reducing the process variation requires an understanding of the common causes affecting the process and their respective effects – a much more involved activity.
While you are working on improving your process you are still producing nonconforming product. Until such time as you achieve the ideal state for your process, you must screen every unit or lot before shipping product to your customer – a 100 percent inspection and sort. This should be treated as a temporary stop-gap measure. You must recognize it as an imperfect quality control method and be mindful that defectives will escape.
Note: I learned this material from reading Dr. Wheeler’s writings. My post is intended to reflect my current understanding. None of the ideas herein are original to me. Any errors are my failures alone.
- Deming, W. Edwards. Out of the Crisis. Cambridge, MA: MIT CAES. 1991. Print. ISBN 0-911379-01-0
- Wheeler, Donald J. and David S. Chambers. Understanding Statistical Process Control. Knoxville, TN: SPC Press, Inc. 1986. Print. ISBN 0-945320-01-9
- Wheeler, Donald J. The Four Possibilities for Any Process. Quality Digest. 1997. Web. http://www.qualitydigest.com/dec97/html/spctool.html