The Elsmar Cove Business Systems and Standards Discussion Forums How much does Cpk > 1.33 represent in precentage of good pieces?
 Forum User Name Keep Me Logged In Password
 Register Photo Albums Blogs FAQ Registered Visitors Social Groups Calendar Search Today's Posts Mark Forums Read

 Elsmar Cove Forum Visitor Notice(s) Some people have been having Password Reset issues (especially "Hotmail" accounts). If you have a problem, contact the Peachfarm Internet Properties IT techs directly at peachfarmllc {at} neomailbox.ch (Switzerland) and we will resolve your problem(s) within 12 hours.

# How much does Cpk > 1.33 represent in precentage of good pieces?

 Elsmar XML RSS Feed Monitor the Elsmar Forum Sponsor Links Courtesy Quick Links Links that Cove visitors will find useful in your quest for knowledge: International Standards Bodies - World Wide Standards Bodies ASQ - American Society for Quality International Standards Organization - ISO Standards and Information Howard'sInternational Quality Services Marcelo Antunes'SQR Consulting, andMedical Devices Expert Forum Bob DoeringBob Doering's Blogs and,Correct SPC - Precision Machining NIST's Engineering Statistics Handbook IRCA - International Register of Certified Auditors SAE - Society of Automotive Engineers Quality Digest IEST - Institute of Environmental Sciences and Technology
Post Number #1
4th November 2003, 07:47 PM
 tattva Total Posts: 46
Cpk

Hi all!

Could someone help me with the following:

How much does CPK>1.33 represent in precentage of good pieces? Is there a matrix where I can find these values at Cpk>1; Cpk>1.67; Cpk<1; Cpk=0?

Post Number #2
4th November 2003, 08:30 PM
 howste Total Posts: 4,295
I've only got a minute before I've got to go, so I'll just get you started...

First, in order for the statistics to be meaningful, you need to have a normal process that is in control. Now, the percentage can actually be different depending on if the distribution is centered or not. A perfectly centered process with Cpk = 1 will have twice as much (theoretical) nonconforming product as one with a Cpk = 1 that is not close to being centered.

What you need to get the percentages is the Z value(s) and a "standard normal" table to look up the results. When you look at the formulas you find that basically Z = 3xCpk.

I'm attaching a standard normal table - hopefully you can figure it out from here. If not I'll be back later...
Attached Files: 1. Scan for viruses before using, 2. Please report any 'bad' files by Reporting this post, 3. Use at your Own Risk.
 Standard Normal Table.xls (55.5 KB, 640 views)
 Thanks to howste for your informative Post and/or Attachment!
Post Number #3
5th November 2003, 07:19 AM
 Howard Atkins Total Posts: 2,933
Here is a very clear article with a table of the results
http://www.symphonytech.com/feature.htm

Post Number #4
5th November 2003, 12:23 PM
 howste Total Posts: 4,295
Great link, Howard. I was trying to explain how a watch works, when all Tattva asked for was the time...
Post Number #5
5th November 2003, 12:47 PM
 Darius Total Posts: 551
Howard, it's a great link, tanks for the tip

For the ppm to Cpk question, both articles, Capability and Six Sigma look like a check mate (no more can be told).

From Six Sigma article (in Howard's link)

Cpk =0.8406+(29.37-LN(ppm)*2.221)^0.5

or

ppm = EXP(-((Cpk-0.8406)^2-29.37)/2.221)

But keep on mind what Don Wheeler said in te point 8.4 from Advanced Topics in Statistical Process Control, 1995.
Quote:
 "It's impossible to convert a capability ratio into a fraction of nonconforming product without using some probability dsitribution in the convertion... of course , the traditional assumption is that the data are normally distributed.... in most cases the uncertainty in the fraction nonconforming will be greater than the refinement offered by such convertion"
Post Number #6
5th November 2003, 01:26 PM
 Sam Total Posts: 1,444
Quote:
 In Reply to Parent Post by tattva Hi all! Could someone help me with the following: How much does CPK>1.33 represent in precentage of good pieces? Is there a matrix where I can find these values at Cpk>1; Cpk>1.67; Cpk<1; Cpk=0? Thanks in advance!
I'm going to step out on a limb and say that Cpk, in and of itself, will not tell you how many good parts you have. Cpk only measures process centering.

Case in point; I have a process that has a Cpk= .8 , Cp = 1.66, Zmin = 2.386, Spec avg = .125, Process avg = .1197
There are zero nonconforming parts. All data entered was within spec.

Using the Z-value would show a "potential" for approx. .85% nonconforming.
When I want to relate percent defective to number of parts I use PPM.
Post Number #7
5th November 2003, 01:41 PM
 howste Total Posts: 4,295
Quote:
 In Reply to Parent Post by Sam I'm going to step out on a limb and say that Cpk, in and of itself, will not tell you how many good parts you have. Cpk only measures process centering. Case in point; I have a process that has a Cpk= .8 , Cp = 1.66, Zmin = 2.386, Spec avg = .125, Process avg = .1197 There are zero nonconforming parts. All data entered was within spec. Using the Z-value would show a "potential" for approx. .85% nonconforming. When I want to relate percent defective to number of parts I use PPM.
The usefulness of statistics comes from measuring samples and then making inferences about the population. My question is (assuming the process is normal and in control), did you measure every part in the population? If not, then I would guess you really do have nonconforming product, you just didn't happen to find it in the samples you took.
Post Number #8
5th November 2003, 03:29 PM
 Sam Total Posts: 1,444
Quote:
 In Reply to Parent Post by howste The usefulness of statistics comes from measuring samples and then making inferences about the population. My question is (assuming the process is normal and in control), did you measure every part in the population? If not, then I would guess you really do have nonconforming product, you just didn't happen to find it in the samples you took.

I think that's what I inferred in my response when I said the reaults of the Z-value provided the "potential" nonconformances.
In response to Tattvas' question "can Cpk be related to Nonconformances"? I said no, and I stick wth that.

 The Elsmar Cove Business Systems and Standards Discussion Forums How much does Cpk > 1.33 represent in precentage of good pieces?

 Bookmarks

 Visitors Currently Viewing this Thread: 1 (0 Registered Visitors (Members) and 1 Unregistered Guest Visitors)

 Forum Posting Settings You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules

 Similar Discussion Threads Discussion Thread Title Thread Starter Forum Replies Last Post or Poll Vote amiuda Lean in Manufacturing and Service Industries 6 14th November 2014 06:36 PM cpine APQP and PPAP 23 25th April 2013 04:42 PM totty ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 19 15th March 2013 01:05 PM ignatiuswong RoHS, REACH, ELV, IMDS and Restricted Substances 6 20th November 2007 01:52 PM Marc Misc. Quality Assurance and Business Systems Related Topics 16 2nd August 2006 01:49 PM

The time now is 06:19 PM. All times are GMT -4.