Some people like to default to the "half Cpk" approach (report Cpu or Cpl) when forced to calculate capability for a unilateral specification. The problem is, it really does not make any sense. The difference between Cp and Cpk is that Cpk answers the question "is the distribution mean centered?" For a unilateral specification, that is a nonsensical question. The target is usually 0, not the center of the specification tolerance. What a "half Cpk" represents is the distance of the mean from the specification. The problem is, nobody is asking that question...they are asking if it is centered. For those in automotive, refer to PPAP 4th edition section 188.8.131.52 Note that tells you Cpk does not apply to unilateral specifications! See the attachment for further discussion.