To understand Cp and Cpk you must have an understanding of the following terms:
|Specification||Specifications define product requirements. In other words, they define what is expected from an item for it to be usable. Specifications are normally defined in terms of nominal (+/-) tolerances or ranges (low to high. A specification for a piston ring, for example, might specify that the diameter be 74mm +/- 0.05mm. The upper specification limit (USL) is the upper limit of the specified range. Similarly the lower specification limit (LSL) is the lower limit of the specified range. See: Specifications.|
|Standard Deviation||The standard deviation is a measure of variability in a process. Defined as the root mean square (RMS) deviation from average it indicates how much a process can be expected to vary from the average. The standard deviation is normally fixed for a process that is under statistical control and can only be affected by a process change that affects the variability in a process.|
|Mean||The arithmetic average of a group of values.|
The formula for Cp is as follows:
The formula for Cpk is as follows:
Caution: Only after a process is under statistical control, can one safely assume that the mean and standard deviation to have a stable values over time.
Cpk is more widely used than Cp, since it takes into account the mean and the standard deviation in its calculation. Please note that the difference between Cp and Cpk is an indicator of how far the average of the process is from the target specification. When the average of the process approaches the target value, the gap between Cpk and Cp closes. When the average of the specification is equal to the target value, then Cpk is equal to Cp. Cpk can never exceed Cp.
Both Cp and Cpk can be calculated with the generation of descriptive statistic views and histograms.
Viewing Descriptive Statistics