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Statistical Process Control and
Quality Control
Overview sional measurements of the precision needed
for SPC are generally done offline. Obtaining
Statistics is a branch of mathematics deal- the final dimensional stability needed to mea-
ing with the collection, analysis, interpreta- sure a part may take time. For example, amor-
tion, and presentation of masses of numerical phous injection molded plastic parts usually
data. The word statistics has two generally ac- require at least a half hour to stabilize.
cepted meanings: (1) a collection of quantita- The SPC system starts with the premise
tive analysis data (data collection) pertaining that the specifications for a product can be
to any subject or group, especially when the defined in terms of the product’s (customer’s)
data are systematically gathered and collated, requirements or that a product is or has been
and (2) the science that deals with the collec- produced that will satisfy those needs. Gen-
tion, tabulation, analysis, interpretation, and erally municates with a series
presentation of quantitative data. of process sensors and/or controllers that op-
Statistical process control (SPC) is an im- erate in individual data loops (Chap. 7). The
portant real-time online method by puter sends set points (built on which
a production process can be monitored and performance characteristics of the product
control plans can be initiated to keep quality are desired) to the process controller, which
standards within acceptable limits. Statistical constantly feeds back to puter to sig-
quality control (SQC) provides offline anal- nal whether or not the set of points are in fact
ysis of the big picture, such as what was the maintained. The systems are programmed
impact of previous improvements. It is impor- to act when key variables affecting product
tant to understand how SPC operates. quality deviate beyond set limits (1, 13, 559,
There are