Why does Ppk Study focus attention on Ppk and not cp, cpk, or Pp?
The four process capability/performance statistics obey the following inequalities: Ppk < Pp < cp and Ppk < cpk < cp. Ppk is always smaller than the other statistics because it accounts for process centering error and both short term and long term variation. This means that, of the four statistics, Ppk is the most conservative and the most indicative of the true performance of the process.
