Before quality can be built into a product, it is necessary to verify that the selected production process is capable of performing at the desired quality level. this process proving involves some form of trial run or pilot test. as the trial run takes place, managers must replicate as closely as possible the actual operating environment. deviations from a realistic operating environment invalidate results. therefore, the same equipment, the same procedures, the same workers, and the same suppliers should all be used in the test run. the output of the process should then be measured against the standard using rigorous statistical analysis to assess the overall process capability.

the process capability ratio assess the process’ ability to achieve required quality levels. by comparing acceptable tolerances (set by engineering) with the process variation, this ratio indicates whether the process can consistently produce "good" quality parts. the ratio is calculated as follows:

cp = | usl-lsl | where: usl = upper specification limit

6s lsl = lower specification limit