Statistical Process Control (SPC) Resource Center

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 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.