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Unit 14: Statistical Process Control
Statistical quality control provides the statistical techniques necessary to assure and improve Notes
the quality of products. Most of the statistical quality techniques used have been developed
during the last century. Basic steps in statistical quality control methodology are represented in
Figure 14.1, which also lists the output of each step.
14.2.1 Advantages of Statistical Quality Control
When the quality of a product is tested by destructive testing, then 100% testing will spoil
all the products. Under statistical quality control very few products will be destructed in
testing.
It ensures control, maintenance and improvement in the quality standards.
It provides better quality assurance at lower inspection cost.
It reduces the wastage of time and material to the minimum. It reduces the inspection and
manufacturing cost and enhances profits.
14.3 Process Capability: A Discerning Measure of Process
Performance
We introduce now an important concept employed in thinking statistically about real life
processes. Process capability is the range over which the “natural variation” of a process occurs
as determined by the system of common or random causes; that is, process capability indicates
what the process can deliver under “stable” conditions when it is said to be under statistical
control.
The capability of a process is the fraction of output that can be routinely found to be within
specifications (specs). A capable process has 99.73% or more of its output within specifications
(Figures 14.2 and 14.3).
Figure 14.2: A Capable Process: Natural Variation is within Spec
Process capability refers to how capable a process is of making parts that are within the range of
engineering or customer specifications. Figure 14.2 shows the distribution of the dimension of
parts for a machining process whose output follows the bell-shaped normal distribution.
This process is capable because the distribution of its output is wholly within the specific range.
The process shown by Figure 14.3 is not capable.
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