Description :
Process capability and process performance indices are long-established metrics of a process ability to meet customer specifications. A Six Sigma process, in fact, has six process' standard deviations between the process nominal and the specification limits. If the process is centered on the nominal it will have two defects per billion opportunities through random chance.

This assumes, however, that the process follows a normal or bell curve distribution; a situation far more common in textbooks than in the real world. When this assumption is not met, however, we can still calculate meaningful process performance indices that reflect the process' true ability to meet specifications.