Six Sigma: Whose idea was it, anyway?
Speculation about origins
How does a term like “Six Sigma” migrate
from a purely statistical application to mean an entire program? Whose idea
was “six sigma quality,” anyway?
A recent forum on www.iSixSigma.com discussed the origins of Six Sigma, both
as a program such as that utilized by Motorola and other industry giants,
and as a statistical term that relates to process capability.
English is a dynamic, living language, and terminology often migrates from a
specific application to a wider use. For example, brand names such as
“Kleenex®" or “Scotch® tape” are so widely recognized that they
are used generically to refer to tissues or cellophane tape even when they
are not actual Kleenex or Scotch brand products. In a sense, the Six Sigma
reference was applied to a specific program, where it has stuck. But of
course that program is derived from an understanding of six sigma as a
statistical term, and of its role in producing products and services of
consistently high quality.
One of the contributors to the iSixSigma.com forum discussion, Michael
Ervick, was knowledgeable about both past and present applications of the
term. He attributed the original—or at least an early—use to W. Edwards
Deming in a book published in 1925, Biometrika. Ervick also noted that it
appeared in Japanese in “Theory of Process Capability and its
Applications” (JUSE Press, 1975). Engineers at Boeing in the 1950s and
Hewlett Packard in the 1960s, he says, “get credit for applying it to the
reliability measures of individual components to measure the impact on
entire systems. As a measure of process capability, it was part of TQM’s
statistical process tool box.” He has promised to keep looking for the
term’s “discovered” birth place in the statistical world.
Certainly, Six Sigma’s application as a program has its history in the
archives of those industries that utilized the statistical measure to assess
and improve their systems. But why six sigma, rather than, say, 3 or 12? The
answer lies in an understanding of process capability itself. Process capability is the “range over which the natural variation of a
process occurs as determined by the system of common causes.” It
represents the ability of a system to produce a product that consistently
meets specifications (James R. Evans and William M. Lindsay, The Management
and Control of Quality, West Publishing, 1993, p. 436. If the natural
variation of a process is larger than the specification, it will be
impossible for that process to meet specifications even when it is in
control. “Six sigma” quality means that while the natural variation is
defined by 3 sigma, the design tolerance should be 6 sigma—or twice as
large as the natural tolerance.
Specifications and process capability link design, manufacturing, and
quality, and it is this link that is the underpinning for Six Sigma, the
program. Motorola, among others, decided that 3 sigma was not good enough to
assure consistently high quality. Six sigma tolerance means that 99.99966%
of the yield will meet design specifications.
To engage in further discussion about this concept and its origins, you may
want to visit http://www.iSixSigma.com/forum/showmessage.asp?messageID=14547