Why Cp should always be greater than Cpk?
Hi! It may be easier to understand it seeing it this way:
6σ is used to represent the process variation (in a normally distributed, controlled process). That is 3σ behind and beyond the process mean.
If you divide your specification range, by the process variation (6σ), you get the Cp; i.e. how many times your process variation is able to fit within specification. However, that alone does not mean that your process is within spec, it only shows how much your process is able to fit within spec if your process is exactly centered within specifications.
For Cpk, instead of comparaing the whole ranges (specs and process variation); it divides them to compare each half: left and right from the process mean. This means that for Cpk, you will be dividing each half and see how many times the upper side of the process variation fits in the upper half of the specification; and then, the lower side of the process fitting in the lower side of the specification. The lowest value of them will be your Cpk.
That is why in a best, ideal, perfect scenario, when yout process mean is exactly at the center of your process specification range, the Cpk value will be the same as the Cp; any offset from the center will cause the Cpk value to be lower than the Cp.
Hopefully this makes a little more sense. Any applied statistics book could guide you through all the mathemathics and considerations needed for the analysis, which are very important to use this tool properly.
Thanku so much for your valuable response
sir i have one question again can you tell me, which limit we have to consider USL & LSL limit for ex. our specification limit 90-110. is that USl or LSL value.
The USL is the Upper Specification Limit, which corresponds to the highest allowable value of your specification range (in your example it would be the 110).
The LSL is the Lower Specification Limit, which corresponds to the lowest allowable value of your specification range (in your example it would be the 90).
Does this answer your question or did I missunderstand your question?
Thanku so much sir. I have clearly understood all the queries related to APQR.