“It could be argued that the use of and C_pk (with sufficient sample size) are far more valid estimates of long and short term capability of processes since the 1.5 sigma shift has a shaky statistical foundation.” Eoin

“C_pk tells you what the process is CAPABLE of doing in future, assuming it remains in a state of statistical control. tells you how the process has performed in the past. _pk, because the process is not in a state of control.