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:
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,