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Quality Assurance by D. H. Stamatis

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12
Statistical Process Control (SPC)
Overview
Statistical process control (SPC) is a methodology for understanding,
analyzing, and improving the process. By denition, it requires an under-
standing of its components, which are
Inputs: Material or data transformed by process to create output.
The inputs are manpower, machine, method, material, measure-
ment, and environment (5M&E).
Process: Any activity that transforms and adds value to the inputs to
result in the desired outputs.
Outputs: The result of transformation of inputs to the desired result.
Feedback: The information from the process or customer that is
taken into consideration for adjustments of the inputs or process.
Model of SPC
Many books in the market have dened, explained, and shown different
implementation plans for an effective SPC program in many industries.
Classical references are Feigenbaum (1961), Grant and Leavenworth
(1980), Ishikawa (1982), Juran and Godfrey (1999), and Shewhart (1980).
In this chapter, we are not focusing on the mechanics of SPC, but rather
on the rationale and an overview of the SPC philosophy. Readers who
are interested in the actual mechanics and calculations of control charts,
capability, and all the details of implementing an SPC program in their
organization are encouraged to see Gitlow et al. (1989), Gulezian (1991),
Wheeler (2010), Montgomery (1985), Stamatis (2003), and others in the
literature.
140 Quality Assurance
The classic SPC model for a generic process is shown in Figure 12.1. The
reader will notice that the product or service goes directly to the customer with-
out any kind of handling. This is what is called the rst time throughput capabil-
ity. Obviously, the product development and production is the PROCESS. We
include product development because that is also a process and it may have
issues with variability as well. The products or services are the outputs of the
process that the customer is willing to pay. That presupposes that all the speci-
cations have been met. The feedback is given by the customer to both inputs
and the process itself, so that improvements may be made as soon as possible.
A much more detailed discussion on this is given by Stamatis (2003).
These four items (inputs, process, outputs, and feedback) are essential in
any endeavor of SPC as they focus on improvement of the process rather
than the product. The emphasis is on the following:
Describe components of a process
Document a process
Describe features/applications of process improvement tools
Describe the concept of variation
Identify opportunities for process improvement
Describe statistical measures of location, dispersion, and skewness
Assess process performance in terms of capability indices
These items identify a systematic approach that must be managed by man-
agement. The systematic approach is to
Understand the process from start to nish
Dene the process goals
Understand how to achieve the dened goals
Focus on key elements (indicators) of the process
Monitor, evaluate, and improve the process
On the other hand, the management approach is a way to dene, develop,
and cascade to the organization the overall strategy of continual improvement.
Voice of the process
Inputs
Product
development
and
production
Products or
services
Customers
FIGURE 12.1
Classic SPC model.

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