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Chapter 19: B. Statistical Process Control 261
Special causes of variation can be detected through simple statistical techniques.
These causes of variation are not the same in all operations. The detection
of special causes of variation and their removal is usually the responsibility of
someone who is directly connected with the operation, although management
is sometimes in a little better position to correct them. The resolution of a special
cause of variation usually requires local action.
The extent of common causes of variation can be indicated using simple
statistical techniques, but the causes themselves need more exploration in order to
be isolated. It is usually the responsibility of management to correct the common
causes of variation, although other personnel directly involved with the operation
are sometimes in a little better position to identify such causes and pass them
on to the management for appropriate action. Overall, the resolution of common
causes of variation usually requires action on the system.
We should note that about 15 percent (or according to Deming, six percent) of
industrial process troubles are correctable by local action taken by people directly
involved with the operation, while 85 percent are correctable only by management’s
action on the system. Confusion about the type of action required is very
costly to the organization in terms of wasted efforts, delayed resolution of trouble,
and other aggravating problems. It would be totally wrong, for example, to take
local action (for example, changing an operator or calibrating a machine) when in
fact, management action on the system is required (for example, selecting a supplier
that can provide better and more consistent raw material).
2. CONTROL LIMITS VERSUS SPECIFICATION LIMITS
Define, describe, and distinguish between
these types of limits. (Application)
Body of Knowledge IV.B.2
It is not very uncommon that confusion about specification limits and control
limits exists in the minds of shop floor workers, technicians, inspectors, engineers,
and managers. However, we must make it clear that specification limits and control
limits are two entirely different concepts and there is no relationship between
them. Moreover, there is no analytical relationship between control limits and
specification limits.
Control limits are defined by the process variability, which is usually determined
by the process standard deviation s, whereas specification limits are set by
the manufacturing team, management, or the customer. The most important point
worth noting is that specification limits are used to evaluate individual values
whereas control limits are used to evaluate sample values.
Part IV.B.2