Capability Indices

In IFS/Quality Management – Capability Indices, you can:

• Calculate C_p and C_pk for any variable data point in the Control Plans.
• Display calculated C_p and C_pk values converted to the corresponding defective rates expressed in parts per million (ppm).
• Display your capability indices in graph form for further analysis.

A process is in statistical control (“in control”) when all special causes of variation have been eliminated and only common causes remain, i.e., observed variation can be attributed to a constant system of chance causes. Since a process in statistical control can be described by a predictable distribution, it is possible to estimate the proportion of parts that are within specification limits. Capability can be described as the natural fluctuation of a key characteristic or process compared to the engineering specifications. Capability literally means "capability to meet a specification."

The two most commonly used Capability Indices are C_p and C_pk:

Where USL = Upper Specification Limit, LSL = Lower Specification Limit and

               n = number of measurements        x_i = measurement data i

C_pk takes into account whether the process is centered or not. This is not the case
with C_p.

If the process is perfectly centered then C_pk = C_p, otherwise C_pk < C_p.

The table below shows the relation between various capability values and the corresponding number of nonconformities (defective rate). 

Typical capability requirements are C_pk and C_p values > 1.33