Process Capability is a relatively simple statistical measure which provides an estimate on the level of process outputs which will be within permitted specification limits.

It provides a comparison between the output of a process versus the process specifications.

The process capability measure therefore allows comparison between desired levels of process capability and the actual performance levels of a process. Where a process is “acceptable as is”, then controls methods such as Statistical Process Control can be applied to monitor the process, where the process is not capable and not meeting desired levels of performance, then action can be taken to investigate and have process improvements implemented to achieve the desired capability levels.

Process capability is a measure obtained by taking a representative sample of process output, performing a statistical analysis and using the results obtained to determine future expected process yields.

Process Capability will provide a single number which details the ability of a process to consistently provide output that will be within required specifications.

In order to measure process capability a process must be “stable”, i.e. there can only be common cause variation present within the process. Common cause variation is variation around the process mean (average), arising from variation which is naturally inherent within the process, it is not caused by some special causes.

A process is Capable if the outputs produced are predictable to be within specification.

A process is stable if it is only influenced by common causes of variation.

You don’t actually need to know the process specifications to determine process stability but you must know the specifications to determine capability.

What a process would be capable of, if it were stable.

The outcome of a Process Capability study is a single metric, which provides an indication of the ability of a process to consistently provide output which is within required specifications.

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