## 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 organization.
- 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 per billion.
- 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 Z-values.

- 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.