Process capability compares the voice of the process, its spread and centering, with the voice of the customer, the specification limits. It is normally summarized by indices such as Cp and Cpk or by a predicted defect rate in parts per million. A capability number only means something for a process in statistical control, and it assumes the data follow the distribution used in the calculation, usually the normal.
Before we release the tool to production I want a process capability study, 30 lots minimum, with the chart showing it's stable.
Run capability on an unstable process and the number is a snapshot of one bad week, not a prediction. Get the control chart clean first.
Also heard as
- capability
- process capability study
- capability analysis
Related terms
Cp
Process capability ratio: the specification width divided by six process standard deviations, ignoring centering.
Cpk
Process capability index that accounts for centering: distance from the mean to the nearest spec limit in units of 3 sigma.
Ppk
Process performance index: a Cpk-style figure computed with the overall standard deviation, capturing long-term variation.
Specification limits
Acceptance boundaries for a product characteristic set by design or customer requirements, labeled USL and LSL.
Sigma level
Process quality expressed as the number of standard deviations that fit between the mean and the nearest spec limit.