Six Sigma Capability
Six Sigma is a broad business approach to drive defects produced
by all processes down into parts per million levels of performance.
This means it’s really about improving the process capability for
all critical-to-quality (CTQ) characteristics from all processes in
The goal in a Six Sigma organization is to
achieve defect levels of less than 3.4 parts per million for every
process in the organization and for every CTQ characteristic
produced by those processes.
Six Sigma has been accepted to mean a
4.5-sigma process, not “true six sigma” process.
A process that
operates with “true six sigma” performance takes up 50% of the
specification if centered. This gives it a Cpk and a Cp of 2.0. A
process such as this will produce defects at a rate of only ~2 parts
Six Sigma professionals have allowed for the process
to drift by up to 1.5 standard deviations from the mean. So if we
have a process with a Cp = 2.0 but allow for a 1.5s drift, then we
have the equivalent of a 4.5 sigma process. That is, the mean will
be 4.5s from the specification limit at the edges of the drift. A
4.5 sigma process yields a 3.4 ppm defect level.
Instead of Cp and
Cpk, some Six Sigma organizations report capability in terms of
The Z-values represent the number of standard deviation
units the mean is away from the specification limits.
Zl is the
distance from the mean to the lower spec and Zu is the distance from
the mean to the upper spec.
Zl equals 3 times Cpl and Zu equals 3
times Cpu. For example, if the Cpl of a process was 1.5, the Zl
would be 4.5.