Process Capability Index - Cp


In process improvement efforts, the process capability index or process capability ratio is a statistical measure of process capability: the ability of a process to produce output within specification limits. The concept of process capability only holds meaning for processes that are in a state of statistical control. Process capability indices measure how much “natural variation” a process experiences relative to its specification limits and allows different processes to be compared with respect to how well an organization controls them.

If the upper and lower specification limits of the process are USL and LSL, the target process mean is T, the estimated mean of the process is μ and the estimated variability of the process (expressed as a standard deviation) is σ, then commonly accepted process capability indices includes the formula shown here.

The formula estimates what the process is capable of producing if the process mean were to be centered between the specification limits. Assumes process output is approximately normally distributed.

Related formulas


Cpprocess capability index (dimensionless)
USLpper specification limit (dimensionless)
LSLlower specification limit (dimensionless)
σstandard deviation (dimensionless)