In order to appropriately measure the capability and performance of a process, that process must...

A process must be stable before its capability and performance can be meaningfully assessed. Stability means the process is operating under statistical control, with only common-cause variation present and no unresolved special causes distorting the output. This is essential in the Measure Phase because capability indices and performance measures assume that the underlying process behavior is reasonably predictable. If a process is unstable, the calculated capability may be misleading because the variation and centering observed today may not represent what the process will do tomorrow. A process does not need to be perfectly centered or exactly on target before capability is calculated, although those factors affect the resulting indices. It also does not have to be measured only with variables data, since capability concepts can be addressed in broader ways depending on the context. The first requirement, however, is process stability. In Six Sigma, this is why control-chart evaluation and understanding of variation come before formal capability interpretation. Therefore, the correct answer is D, stable, because predictability is the foundation for valid capability and performance assessment.