Process capability index (Cpk) is a statistical tool, to measure the ability of a process to produce output within customer's specification limits. In simple words, it measures producer's capability to produce a product within customer's tolerance range.
When a large amount of data is to be evaluated, one takes the help of statistics. It is known that when the variation in the data is due to common causes only, the frequency distribution is a bell curve and from that various attributes are arrived at for evaluation.