You can use a process-capability study to assess the ability of a process to meet specifications.

During a quality improvement initiative, such as Six Sigma, a capability estimate is typically obtained at the start and end of the study to reflect the level of improvement that occurred.

Several capability estimates are in widespread use, including:

• C_p and C_pkare process capability estimates. They show how capable a process is of meeting its specification limits, used with continuous data.
• “Sigma” is a capability estimate typically used with attribute data (i.e., with defect rates).

Capability estimates like these essentially reflect the nonconformance rate of a process by expressing this performance in the form of a single number. Typically this involves calculating some ratio of the specification limits to process spread.

Contributed by Keith M. Bower, a statistician and webmaster of www.KeithBower.com,