This is one of the important tools of (SPC) Statistical Process Control

Process Capability Index Cp

This is one of the important tools of (SPC) Statistical Process Control. Process capability is the measure of the ability of process to meet its customer specified requirements. The process is said to be capable when the output confirms to desired specifications. This is defined by Process Capability Index Cp which is a dimensionless number that indicates its capability limits defined by customers.

Cp - Process Capability Index

Cp is the ratio of customer’s specified tolerance of characteristics divided of the natural process variation. The process should be in a state of statistical control.

Cp = VOC (Voice of the Customer)

VOP (Voice of the Process)

Cp = Engineering Tolerance

Natural Process Variation

Cp = USL – LSL

Six Sigma

USL is upper speciation limit
LSL is upper speciation limit
Six Sigma is the natural process variation

Ideally, a capable process should be able to meet customer specification requirements, in that case Cp should have minimum value of 1. In industry Cp = 1.33 is considered to be a normally accepted value.

The most important contribution of Cp Index is to support in studying the process behavior and its capability to deliver right quality product specified by customers. This ensures a better quality output in the manufacturing.

The Cp index has a limitation of not able to clearly define the central tendency of process relative to its specifications.