Interpretation & Calculations
Process Capability refers to the ability of a process to consistently meet the demands of its customers. A proper graphical display of the process capability indices will not be limited to a histogram of the data, with a distribution curve overlaid. Instead, the control chart used for calculating process sigma, and verifying the process is in control, is also necessary. Analysts should never evaluate the process capability indices without the context of a statistical control chart. Process capability indices attempt to indicate, in a single number, whether a process can consistently meet the requirements imposed on the process by internal or external customers. Much has been written about the dangers of these estimates, and users should interpret process capability calculations only after understanding the inherent limitations of the specific index being used, most especially the requirement that the process be in a state of statistical control for the process capability index to have any legitimacy.
Process Capability Calculations:
Learn more about the SPC principles and tools for process improvement in Statistical Process Control Demystified (2011, McGraw-Hill) by Paul Keller, in his online SPC Concepts short course (only $39), or his online SPC certification course ($350) or online Green Belt certification course ($499).