How do I calculate Cpk with a single Control Limit?

[Bob in KY]

Is it possible to come up with a CPK for a part with a single control limit. We take readings on the part and the minimum reading allowed (the single control limit) is 25, however it can be above 25, just not below 25. Therefore we have readings anywhere from 25 to 55. How would I go about calculating a CPK with the single Control Limit of 25? I'm sure this is very simple for most everyone, but I'm very new to the Quality Assurance program. Any help would be greatly appreciated. Thank you all.

[Atul Khandekar]

Welcome Bob in KY!

I think you are confusing between Control Limit and Specification Limit. In this case 25 appears to be the Lower Spec Limit. (Is the upper spec 55? If yes, then you clearly have two sided specs.)

This topic has been discussed before:
Calculating Cpk on a single sided tolerance

CP and CPK for product w/ only LSL

[Bob in KY]

Thank you Atul Khandekar, I now see that I cannot get a Cpk and must use a Cpl. Thank you again.

[cleverfox]

Hi Bob,

I have several parts in which there is only one spec limit. CPK is calculated by "x(bar) - LSL / 3s".

Good luck,
-Fox

[Bob in KY]

Cleverfox, pardon me for being ignorant on the subject but what is the "s" in the "3s", of x(bar) - LSL / 3s. Thanks.

[Bob in KY]

would the "s" in "3s" be Standard Deviation?

[cleverfox]

Bob,

The "s" is for standard deviation; calculated as s = R(bar)/ d2. The "d2" is a constant and its value is found on a chart in the AIAG SPC reference (or other SPC guides) manual.

-Fox

[ysdennis]

Your 25-55 should be the sampling amount,right?.
Actually,it would be stlll OK if over 55. The reason why we have to collect 25 above is that the sampling distribution of X-bar will be approaching to Normal distibution. This is derived from "Law of large number" in statistics.