The term sigma is taken from a letter in the Greek alphabet and is used
in statistics as a measure of variation.
For a business or manufacturing process, the sigma value is a
metric that indicates how well that process is performing. The
higher the sigma value, the better.
More specifically, sigma measures the capability of the process
to perform defect-free work. A defect is anything that results in
customer dissatisfaction. With Six Sigma the common measurement
index is "defects per unit," where a unit can be virtually anything-for
instance, a component part, piece of material, line of code,
administrative form, time frame, or distance.
The sigma value indicates how often defects are likely to occur. As
sigma increases, cost and cycle time go down while customer
satisfaction goes up.
Most companies operate at about three sigma, conforming to the
US Governmental quality standards of 99% as established during
World War II.
Working at 99% defect-free, however, means:
At least 200,000 wrong drug prescriptions each year
Two short or long landings at major airports each day
5,000 incorrect surgical procedures every week
20,000 lost articles of mail per hour
Unsafe drinking water for almost 15 minutes each day
No electricity for almost 7 hours each month
50 dropped newborn babies each day
Six sigma is the goal, which means products and processes will
experience only 3.4 defects per million opportunities, or
99.99966% good. When we say a process is at six sigma, we are
saying it is the best in its class.
Obviously, facility with statistics is beneficial in understanding Six
Sigma, but the real emphasis of the methodology falls on business
management. The fundamental objective of Six Sigma is customer
satisfaction through continuous improvement in quality. |
