Capability Charts

The process capability chart is the frequency histogram and distribution of the process mean data and is used to analyze process capability. This chart is drawn with center line, lower specification limit and upper specification limit indicators in place. Interpreting capability charts is typically made with the Cp, Cpk, kurtosis and skewness indices.

Refer to the following for more information:

Upper Specification Limit and Lower Specification Limit

Upper specification limit (USL) and lower specification limit (LSL) specify the requirements of the product, and so are customer driven. Make the target value the midpoint between USL and LSL.

Cp Index

The inherent process capability (Cp) is the ratio of tolerance to 6-sigma. Essentially this index indicates whether the distribution would fit inside the USL and LSL limits. Its meaning is defined as follows:

• Cp > 1.0 - Indicates the process variation will fit within the specified limits (USL and LSL) and therefore, is capable.

• Cp < 1.0 - Indicates the process is not capable.

Cpk Index

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.