Cp = 
The Cp ratio express how the width of the specification limits relates to the six sigma range of the production. Note that 6 sigma limits include 99.7% of the production as modeled by a normal curve.
Unlike the Cpk, the Cp says nothing about the relative position of the production within the spec limits.
= 0.51. This means the spec limit range is about half the production
range. A desirable Cp ratio is 
= 1.33, expressing that the spec limits are 133% wider than the
production. This works out to 2 standard deviations wider than
the production spread. The PCI ratio is the reciprocal of the Cp ratio (
).The Cpk, the Cp and the PCI all express the same concept. Since there are three different ratios in use, it would be a good idea to clarify each time how you are calculating the ratio you use.
#37 Calculate the Cpk ratio for the following data collected on samples of resister pull strengths. The procedure is under statistical control. The specification limits are 1.0 and 6.0. Draw a normal curve with z score intervals, the process center, and specification limits shown.
Remember that we are re-creating a normal distribution of individuals.
A histogram of the sample means will not help estimate the percentage
of individuals out of specification, nor place individuals in
relation to the specification limits.
sample mean frequency
1.5 1
1.9 2
2.3 3
2.7 6
3.1 6
3.5 7
3.9 8
4.3 5
4.7 3
5.1 1
The average range = 1.95 and sample size n = 5.