What is the difference between cp and cpk

Mean The arithmetic average of a group of values. Cp The Cp index is calculated using specification limits and the standard deviation only. This index indicates, in general, whether the process is capable of producing products to specifications. No information on the ability of the process to adhere to the target value is included in this index. The formula for Cp is as follows: CpK This Cpk index is calculated using specification limits, the standard deviation, and the mean. Process has achieved 6 sigma capability.

But as we see my results are towards Lower Specification Limit. All are skewed to left of the plot. Can anyone please help me in calculating the Cp and Cpk of this Process. Plese help me in resolving this its urgent…….. If it was my process, I would take more samples e.

I would also try to increase the precision of your measurements, as you could expect the difference between 1. So in short, the issue probably lies in your SD figure, not in your calculations but your method. The analogy we used at Motorola in the mid-eighties when Six Sigma was devloped: Imagine we are the road crew painting a series of white stipes that form dotted lines down the center of the highway.

CP — The more the slight variations, meaning the looser our process control becomes people fatigue in the hot sun, or because the machine occasionally sputters and splatters the more likely the stripe will soon become unacceptably long, short, or wide, thereby creating a statistical number of rejects per thousand, or per million, etc.

And… CPK — The less centered we are, as our vision starts to blur, or the truck wobbles to one side the sooner we can expect to reduce the lane size so that it is too small for cars to fit, or maybe we will even run off the road!

I highly recommend all of you to quit using software. You need to understand the fundementals of this to even being to know what any answer means. All mathematical calculations should be performed by hand only using a calculator. By doing this you will understand what the answer means. Very apt articulation of the subject matter — the concept has been explained quite lucidly. Thanks a ton. Short version of this is. Then it is up to You to define what is long term, short term and if to use subgroups or individual data points.

So if You collect data one Monday morning, say one sample every 15 min for 4 hours You will get 13 samples. Sigma within based on table values or sub group calculations is used for Cp and Cpk calculations. Sigma overall is used for the Pp and Ppk calculations. The same goes for if You would have collected the data for a whole week, say one sample on Monday one on Tuesday and so on.

Same calculations but different analysis. And logically the meaning of the four different capability indexes are; Ppk is what the customer sees. Pp is what Ppk would have been if the process were centered.

Cpk is what the Ppk will be if the process is stable no change between subgroups mean. Cp is what Ppk will be if the process is stable AND centered. Automotive Industrie requires a Ppk better than 1,67 and Cpk better than 1,33 or Ppk better than 2,0 and Cpk better than 1,67 f. Very Useful Article. It helps me a lot in understanding the meaning and concept of Process Capability.

In six sigma, histogram indicates specification limits are kept 6 standard deviations of both side. But normally distribution of process with in 3-sigma level.

Please explain this doubt. Thank for your sharing and explain about Cp and Cpk. Was really happy to come across this article while doing a bit of research.

Well done! I especially like the variety of quoted interpretations. Simply and excellent explanation of process measuring methodology. It explains clearly each and every stage. Congratulations for this content! Thank you a lot! Thanks for it. But I want one example with distribution. Do the cpk of 3 runs must have the same value? Are they acceptable? What I have to do to correct if that is not acceptable? Another way to see the difference is Cp and Cpk use the within group variation and Pp and Ppk use total observed variation in their calculations.

Cpk should not be over 2 value. If it is over 2 , have to check n review again the specification vs actual results. You must be logged in to post a comment. Please Sign in Register. By iSixSigma-Editorial. The measure of the strength of the linear association in a correlation analysis. A collection of individual pieces from a common source, possessing a common set of quality characteristics and submitted as a group for acceptance at one time.

Data are factual information used as a basis for reasoning, discussion or calculation; often this term refers to quantitative information. The mean is the average data point value within a data set. You Might Also Like. Capability 24 Cpk Comments Walt Crompton Excellent summary!

April 13, at am - Log in to Reply. May 25, at am - Log in to Reply. Allan Very Useful Article.. How can I calculate Cp or Cpk in this scenario? Is this related to SPC? Table, an empirical function? We have a few resources on data sampling techniques and sample size here. Hi, I have a specification, which sets 6 limits for a production process. I have no problem for creating individual measurements control chart, but cannot figure out how to find my upper and lower limits for range chart based on the above conditions.

I have a manager asking me to provide CPk data for a special design characteristic feature. We typically do not use CPk to track these, because our processes require that we measure every part and document our results. Special design characteristics have a very small tolerance band, thus the CPk value will be naturally low.

Am I thinking about this correctly? And if so, can you give any advice on how I can simply explain to this manager why his request is unreasonable? So i need to adjust the machine as to ensure the part result pass assuming there have machine issue.

Do i need to remeasure all those 29 parts after adjusting the machine? The values for Cpk and Ppk will converge to almost the same value because sigma and the sample standard deviation will be identical use an F test to determine. I want to assess the impact on Cp when I chop a tail of a distribution which is normal to begin with. If you have a ready reference please let me know. I am thinking of the following: say we have normal distribution data with a given sigma and mean.

Now I want to see if I remove the lower 10th percentile and the top 90th percentile of the data what will happen to my cp and cpk. I am thinking of the following approach 1. Knowing 10 and 90th percentile work through the z value and come up with the corresponding x values for these percentiles 2.

Make these new x values as the new say 6 sigma values and compute the new sigma — this is trivial but it is a step 3. Compute the new cp and cpk using the same mean but new sigma. The question is: is there a mathematical formulation that you can point to vs doing this manual operation. Is this approach valid or there are other approaches? Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed. Process Capability This is a long article, but I thought it was important to keep Cp and Cpk together. Before We Begin! Cp Cpk vs Pp Ppk. That was poorly centered! Capability Index. Unlock Additional Members-only Content!

To unlock additional content, please upgrade now to a full membership. Upgrade to a Full Membership If you are a member, you can log in here. Now check your email to confirm your subscription. There was an error submitting your subscription. Please try again. First Name. Email Address. We use this field to detect spam bots. If you fill this in, you will be marked as a spammer. Ted Hessing. Comments Hi, if I have a set of data where the subgroup size is different, how should I determine which d2 value to be used for the Cpk calculation?

How was out of spec percentage 2. Yes, Parag. See the notes that Joanna Han left above. How can I help? No idea what you are asking for. What is the value of Cpk for six sigma process. Read the article. What do you think, Prabin? Yes, I can. What have you tried so far? Hi Chandana, What do you think would be appropriate and why? Where do you see that? Thank you so much. Do they help? Best, Ted. Thank you. Can you elaborate? Can you add a bit more detail? Hi, Sorry for my unclear question before.

Thanks for the comments. The sample size was homogeneous. The population will slightly change every time. The testing method will be the same each time. However, I will consider the other sources for my future analysis plan Thanks again. Then start applying your substitutions. Begin with the given, i. From there, see how to elegantly interconnect the Z score in the Cpk formula.

Dear Sir, I wish to sign up to your newsletter. Thanks a lot! Alex, what have you tried here? I have the same question when preparing for ICBB — all solutions do not make sense. Which is it? Sabarish, can you show your calculation here?

All dimensions are in mm. Hi Ted: I appreciate that you continue share the six sigma information to me. I think that I can discuss with you , could you please kindly to answer? And we can transfer cpk to sigma level because But how about a attribute data?

This is a good question. Hi Dany, I only have the capacity to answer these kind of questions in the paid forum. Hi Samy, This is a great question. We hope you will be able to respond positively to our request. We will cite appropriately.

Jim Berto James J. Let me see if I understand what you are asking. What action you likely to take? Hi Hasti, What have you tried so far? I am working with them and need to know. Unlike the Cp, the Cpk is measured using standard deviation, specification limits and mean.

Another difference between Cp and Cpk is that while Cp gives a description of form, Cpk provides that of form and location. It is important to note that the difference between Cp and Cpk indicates how far the average of the process and the target specification are from one another. When the average of the process is near the target, the Cp and Cpk gap closes, and when the specification value has an average equal to the target value, Cp and Cpk become equal.

Cp and Cpk are used for process capability when a process is under statistical control. It is used mainly for mature processes. Process capability makes use of the process sigma value which is determined either from the range, moving range, or sigma control charts.