Applied Statistics and Probability for Engineers

16-3 STATISTICAL PROCESS CONTROL 597

2. Statistical methods can be used to determine the capability of a manufacturing
   process. Statistical process control can be used to systematically improve a process
   by reducing variability.


3. Experimental design methods can be used to investigate improvements in the
   process. These improvements can lead to higher yields and lower manufacturing
   costs.


4. Life testing provides reliability and other performance data about the product. This
   can lead to new and improved designs and products that have longer useful lives and
   lower operating and maintenance costs.
   Some of these applications have been illustrated in earlier chapters of this book. It is
   essential that engineers, scientists, and managers have an in-depth understanding of these
   statistical tools in any industry or business that wants to be a high-quality, low-cost pro-
   ducer. In this chapter we provide an introduction to the basic methods of statistical quality
   control that, along with experimental design, form the basis of a successful quality-
   improvement effort.

16-2 STATISTICAL QUALITY CONTROL

The field of statistical quality control can be broadly defined as those statistical and engineer-

ing methods that are used in measuring, monitoring, controlling, and improving quality.

Statistical quality control is a field that dates back to the 1920s. Dr. Walter A. Shewhart of the

Bell Telephone Laboratories was one of the early pioneers of the field. In 1924 he wrote a

memorandum showing a modern control chart, one of the basic tools of statistical process

control. Harold F. Dodge and Harry G. Romig, two other Bell System employees, provided

much of the leadership in the development of statistically based sampling and inspection

methods. The work of these three men forms much of the basis of the modern field of statis-

tical quality control. World War II saw the widespread introduction of these methods to U.S.

industry. Dr. W. Edwards Deming and Dr. Joseph M. Juran have been instrumental in spread-

ing statistical quality-control methods since World War II.

The Japanese have been particularly successful in deploying statistical quality-control

methods and have used statistical methods to gain significant advantage over their

competitors. In the 1970s American industry suffered extensively from Japanese (and other

foreign) competition; that has led, in turn, to renewed interest in statistical quality-control

methods in the United States. Much of this interest focuses on statistical process control

and experimental design. Many U.S. companies have begun extensive programs to

implement these methods in their manufacturing, engineering, and other business

organizations.

16-3 STATISTICAL PROCESS CONTROL

It is impractical to inspect quality into a product; the product must be built right the first

time. The manufacturing process must therefore be stable or repeatable and capable of op-

erating with little variability around the target or nominal dimension. Online statistical

process control is a powerful tool for achieving process stability and improving capability

through the reduction of variability.

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