I
n this chapter you will look at one of the statistical tools used in
manufacturing and industry. The proper use of quality control can
improve productivity, enhance quality, and reduce production costs.
In this chapter, you’ll learn about one such tool, the control chart, that is
used to determine when a process is out of control and requires human
intervention.Statistical Quality Control
The immediately preceding chapters have been dedicated to the identifi ca-
tion of relationships and patterns among variables. Such relationships are
not immediately obvious, mainly because they are never exact for individual
observations. There is always some sort of variation that obscures the true
association. In some instances, once the relationship has been identifi ed, an
understanding of the types and sources of variation becomes critical. This
is especially true in business, where people are interested in controlling the
variation of a process. A process is any activity that takes a set of inputs and
creates a product. The process for an industrial plant takes raw materials and
creates a fi nished product. A process need not be industrial. For example,
another type of process might be to take unorganized information and pro-
duce an organized analysis. Teaching could even be considered a process,
because the teacher takes uninformed students and produces students capa-
ble of understanding a subject (such as statistics!). In all such processes, peo-
ple are interested in controlling the procedure so as to improve the quality.
The analysis of processes for this purpose is called statistical quality control
(SQC) or statistical process control (SPC).
Statistical process control originated in 1924 with Walter A. Shewhart,
a researcher for Bell Telephone. A certain Bell product was being manufac-
tured with great variation in quality, and the production managers could not
seem to reduce the variation to an acceptable level. Dr. Shewhart developed
the rudimentary tools of statistical process control to improve the homoge-
neity of Bell’s output. Shewhart’s ideas were later championed and refi ned
by W. Edwards Deming, who tried unsuccessfully to persuade U.S. fi rms to
implement SPC as a methodology underlying all production processes. Hav-
ing failed to convince U.S. executives of the merits of SPC, Deming took his
cause to Japan, which, before World War II, was renowned for its shoddy
goods. The Japanese adopted SPC wholeheartedly, and Japanese production
became synonymous with high and uniform quality. In response, U.S. fi rms
jumped on the SPC bandwagon, and many of their products regained mar-
ket share.488 Statistical Methods