Nonconforming units

Nonconforming units describe the number of nonconforming units a process produces, expressed in parts per million.

The number of nonconforming units is an alternative to traditional Cp and Pp indices. They are easily understandable by end-users.

The number of nonconforming units is either:
  • The actual number of nonconforming units in the sample.
  • The expected number of nonconforming units, assuming a normally distributed process quality characteristic with the parameters specified by the process mean and sigma. The process sigma is either the short-term or long-term sigma estimate.
Various indices measure how the process is performing against the specification limits:
Index Description
< LSL The number of nonconforming units that are less than the lower specification limit.
> USL Thr number of nonconforming units that are greater than the upper specification limit.
< SL > The total number of nonconforming units that are outside the specification limits.