The process capability based on the worst case
(Cpk) is similar Cp. This index, however, indicates where the mean
lies in relation to the USL and LSL limits. It is used to
mathematically clarify Cp. Its meaning is defined as follows:

Cpk <
0 - Indicates the process mean is outside the specified limits
(USL and LSL)

Cpk =
0 - Indicates the process mean is equal to one of the
specified limits.

Cpk >
0 - Indicates the process mean is within the specified
limits.

Cpk = 1.0
- Indicates that one side of the 6-sigma limits falls on a
specification limit.

Cpk >
1.0 - Indicates that the 6-sigma limits fall completely within
the specified limits.