Setting the Record Straight with Capability Indices

There is still much controversy and confusion in industry today regarding the use of process capability indices and analysis. A lack of knowledge regarding the underlying variation assumptions and rationale sampling strategies in indices such as Cp, Cpk, Pp and Ppk have added to the confusion and in many cases has led to misapplication of these widely used metrics. This issue has also promoted inconsistency in the assessment of long-term versus short-term capability and has hampered the true characterization of processes which is a critical step in any continuous improvement effort. Capability indices and analysis, when properly applied can impart a wealth of knowledge regarding process performance as well as provide focus for improvement activity through the proper characterization and enumeration of variation. Being able to properly characterize variation if the first step in reducing it.