Typically used in manufacturing, cp and cpk are indices that can reveal at a glance whether a process is producing items within specification or not, and why. This tool allows you to quickly calculate these indices, and a few other basic statistics, on a small sample of measurements. The cpk index is often reported by itself because it will reveal what cp index does while also providing unique information of its own. The cpk index is the 'executive brief' of process capability indexes.

cp Warning

A value of cp below 1 indicates that the population cannot fit within the specification limits because there is too much variability in the process. Typically a simple adjustment to the target value is not sufficient to correct this problem, but rather a fine-tuning of all the factors that contribute to consistency is in order, or even a re-evaluation of the specifications to be sure they aren't unrealistically tight.

cpk Warning

A value of cpk below 1 indicates that some of the population is outside of the specified limits. This could be due to a centering problem, perhaps requiring a simple adjustment of the process towards the target value, or it could be due to the population being too scattered to fit within the specifications (cp<1), or both.

cpk Alarm

A value of cpk below 0 indicates that the population mean is outside of the specification range - over half of the population will probably be out of specification. Time to shut whatever it is down and fix it.

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