It is all about the control of process variables when you want to assure the process output.
Same concept goes with Section I, III, VIII, etc. for QC system audit (Authorization Certificate), Section V for NDE,.....
Consider a QC system of a stamp holder, a welding procedure, a welder, an operator, an NDE procedure, and NDE inspector.......as "process" to transfer "input" into "output".
You want to assure the quality (desired output) of a process, you need to care about the variables.
Refer to ASM Handbook, Volumn 17, Statistical Quality Design and Control or ISO900X.
Quota:
"
A MAJOR REVOLUTION in the industrial sector has been taking place in America during the 1980s as manufacturers
strive to regain the competitive position they once held in the world marketplace. One element of this revolution has
centered around a renewed emphasis on quality, with an approach aimed at preventing defective materials from being
manufactured through improved process monitoring and diagnosis and at designing quality into the product from the very
beginning. The concepts and methods of Deming and others have had a profound impact on the way quality is viewed
from the manufacturing/process perspective. The simple but powerful statistical methods for process control developed by
Shewhart some time ago have been successfully revived and applied on a very broad basis. In the engineering design
arena, the methods of Taguchi, referred to as off-line quality control, have been successfully used for more than 30 years
to provide a sound basis for improved product/process design. From the total system point of view, the concept of
company-wide quality control, which has been practiced in Japan for some time, is now receiving considerable attention.
In particular, recent emphasis has been placed on quality function deployment as a means to transmit customer needs
through the organization both vertically and horizontally.
The work of Shewhart in the 1920s led to a sound approach to the scrutiny of process variation and the diagnosis and
removal of process faults. However, the statistical approach to the sampling of process output prior to shipping to
determine the extent to which it conformed to specifications dominated the quality field from the 1930s through the
1970s. Unfortunately, it has only recently been recognized that this product control approach to quality control
contributes little to the enhancement of competitive position. The recognition that quality and productivity can move
together in the right direction only when the process is evaluated, finding the root cause of process faults and taking
action to remove them, is today reshaping the meaning and intent of quality control.The dramatic change in the meaning and application of quality control as a discipline has both a philosophical and an
analytical side. These two aspects of the problem must be understood together. This article will present some fundamental
elements of statistical thinking and methods for quality design and control. Commonly used techniques employing control
charts and the design of experiments are discussed with illustrations.
"