# How do I calculate Cpk with a single Control Limit?

B

#### 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.

B

#### Bob in KY

Cpl...thank you!

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

C

#### 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

B

#### Bob in KY

cpk

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

C

#### 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

Y

#### 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.

#### Darius

##### Quite Involved in Discussions
I still felt that cpmk could be better estimate for one sided specs. I added a spreadsheet with an example of "with larger variation on target or less variation far away from target.

by the way
This is derived from "Law of large number" in statistics.
is the central limit teorem?....

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L

#### lday38

economic control limits /sample size of 2

I am switching from a sample size of 5 to 2 pcs. I have a history of spc on 5 piece chart. I had calculated economic control limits( infinity qs) on the 5 pc charts. The two pc sample size would be before the operator dumps the parts into a bin. I am usign visual spc software
I would like to hear feedback on anyone using other control limits when the CP and Cpk is large and also any feedback on the type of chart for a sample size of 2.

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