Cp in the acronym Cpk stands for "Capability process" in reference to a statistical process control (SPC) chart. SPC charting is used as part of the qualify control analysis of a manufacturing process.
The Cp and Cpk indices compare the variability of manufacturing processes against predefined upper and lower limits — also known as the tolerance limits.
The Cp index, as its name implies, is an indicator of overall process capability, which can be defined as how well the product conforms to consumer expectations or specifications. Cp describes how often the actual manufacturing processes fall within the predefined tolerance widths. The higher the value, the more efficient the process in question. For example, a Cp of 2 means that the data fits two times within the tolerance limits. This is displayed in graphic form on a chart. However, the Cp index will remain the same even if the data is off center with respect to the middle of the data tolerance range.
The Cpk index corrects for the position of the data on the graph. If the Cpk is equal to the Cp, then the process is ideally set up to operate in the middle of the tolerance range. A high Cpk is indicative of an efficient process and represents a small spread in relation to the tolerance width. A typical Cpk requirement is 1.33 or greater.