Most capability assessments can be grouped into one of two categories: Potential (within) and Overall capability. Each represents a unique measure of process capability. Potential capability is often called the "entitlement" of your process: it ignores differences between subgroups and represents how the process could perform if the shift and drift between subgroups were eliminated. Overall capability, on the other hand, is what the customer experiences; it accounts for the differences between subgroups. Capability indices that assess potential capability include Cp, CPU, CPL, and Cpk. Capability indices that assess overall capability include Pp, PPU, PPL, Ppk, and Cpm.
For example, you inspect equipment at a candy factory, including a machine that fills containers with a certain weight of candy. Candy shipments leave the factory once each week. To assess the capability of this process, you weigh a sample of bags each day for a week; each sample represents a subgroup in your analysis. You observe that the variation within each subgroup is small, but because the subgroup means shift from day to day, the overall variation of weight of all bags is much greater. Consequently, the entire weekly shipment has larger variation in bag weight than the bags produced on any given day. In the chart below, the small distributions represent the distribution of bag weight for each of seven consecutive days. The topmost distribution represents the entire week's shipment, which is an aggregate of the subgroups.
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Overall capability considers the topmost curve: what the customer feels. It is also called actual capability or long-term capability. |
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Potential capability considers the variation within subgroups: how well could your process perform if you eliminated variation between subgroups. It is also called short-term capability. |
Potential capability is only calculated for normal data.
