Within and overall refer to different ways of estimating process variation. A within estimate, such as Rbar/d2, is based on variation within subgroups. The overall estimate is the overall standard deviation for the entire study. Cp and Cpk are listed under Potential (Within) Capability because they are calculated using the within estimate of variation. Pp and Ppk are listed under Overall Capability because they are calculated using the overall standard deviation of the study.
The within variation corresponds to the inherent process variation defined in the Statistical Process Control (SPC) Reference Manual (Chrysler Corporation, Ford Motor Company, and General Motors Corporation. Copyright by A.I.A.G) while overall variation corresponds to the total process variation. Inherent process variation is due to common causes only. Overall variation is due to both common and special causes. Cp and Cpk are called potential capability in Minitab, because they reflect the potential that could be attained if all special causes were eliminated.
To view the formulas for Capability Analysis and Sixpack (Normal), including the formulas for estimating the standard deviation, see Knowledgebase ID 294.
