The output of this measurement is often illustrated by a histogram and calculations that predict how many parts will be produced out of specification (OOS).
This should include the normal variety of production conditions, materials, and people in the process.
With a normal distribution, the "tails" can extend well beyond plus and minus three standard deviations, but this interval should contain about 99.73% of production output.
Therefore, for a normal distribution of data the process capability is often described as the relationship between six standard deviations and the required specification.
The output of a process is expected to meet customer requirements, specifications, or engineering tolerances.