When is cpk good

You are unlikely to produce defective parts even if the process starts to drift. Truncated: A truncated distribution may indicate that out-of-spec parts have been removed from the data set. Inadequate Resolution: Inadequate gage resolution may result in misleading calculations of the average and standard deviation.

Multi-Modal: A distribution with more than one peak may be caused by multiple operating conditions. The ideal measurements therefore will be exactly at or close to the LSL, and in this particular case, we do want measurements to be close to the LSL. Unilateral Specs: Making parts at the nominal will be too risky. The manufacturing engineer will therefore need to select a target value offset from the nominal, so that the tails of the process do not extend beyond the spec limits. In general, the higher the Cpk, the better.

A Cpk value less than 1. A value between 1. But, you should aim for a Cpk value of 2. A Cpk of 2. The second benefit may not be very obvious. Watch a video here to see how a Cpk or Ppk of 4. What is Process Capability Analysis? The basic idea is very simple. You want your manufacturing process to: 1 be centered over the Nominal desired by the design engineer, and 2 with a spread narrower than the specification width. Cp measures whether the process spread is narrower than the specification width Cpk measures both the centering of the process as well as the spread of the process relative to the specification width.

The Basic Calculations Before we get into the detailed statistical caluculations, let's review the high-level steps: 1: Plot the Data: Record the measurement data, and plot this data on a run-chart and on a histogram as shown in the picture on the right.

I originally created SixSigmaStudyGuide. Go here to learn how to pass your Six Sigma exam the 1st time through! View all posts. 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? If I perform a Ppk calculation, is the Ppk value going to be affected by the difference in subgroup size? Are you asking which d2 value to choose if you have multiple subgroups of varying size? My first question was about how to determine d2 for multiple subgroups of varying size, i.

Since we all know that in practice we often inherit data, so this may not be possible. My instinct would be to take the average of the subgroup sizes. Trying to get my hands on a copy now. Hi, I am doing data analysis where the subgroup size is not constant. I was thinking to take the majority or average but I have found that there is formula to determine the sigma. The Shooting at a Target Analogy A good analogy is shooting at a target. If the rounds form a good cluster or grouping in the same spot anywhere on the target you have a high Cp value.

When the you have a tight group of shots is landing on the bulls eye, you now have a high Cpk Cpk measures how close you are to your target and how consistent you are to around your average performance.

A person may be performing with minimum variation, but he can be away from his target towards one of the specification limit, which indicates lower Cpk, whereas Cp will be high. On the other hand, a person may be on average exactly at the target, but the variation in performance is high but still lower than the tolerance band i.

In such case also Cpk will be lower, but Cp will be high. Cpk will be higher only when you r meeting the target consistently with minimum variation. Hi Pavel, it was calculated using a Z Score. Thanks for the question! Sign up or log in for access. Can you solve the problem? The weights of nominal 1-kg containers of a concentrated chemical ingredient are shown in Table 8E. Suppose there is a lower specification at 0.

Calculate an appropriate process capability ratio for this material. What percentage of the packages produced by this process is estimated to be below the specification limit?

Thanks in advance. Hi, 1. Hi, I have a data which contains the quality scores of the individual persons from last 50 weeks Individual scores for 50 persons on 50 weeks. Or any other method will be used? Please suggest.

Currently, I have been measuring the quality for a group of staffs on a weekly basis. Now I have the data history for the last one year. If I want to see the statistical detail for the past one year data which means can I able to say the sigma levels for each staff what method will be used?

Interesting question. Is the sample size homogeneous? Were the same people measured for every test or did the population change over time?

Was the test the same each time? Mostly same people were measured for every test. Sometimes, the new people were added and will be added overtime.

Hard for me to give a straight answer without knowing more details on what kind of analysis you will be looking to do, but here are some thoughts:. Look at what Jeremy did in his case study on using control charts on student test scores. Depending on your use, you might consider an EWMA chart. Hi Thanks very much for the detailed response. Now I will start my analysis with the baseline sigma. However, I will consider the other sources for my future analysis plan. How does a Cpk of 2.

This is a good homework question because it shows the relationship between process capability and quality. Start by listing the Cpk formula and substituting the 2. What are you left with? It is a homework question. He said, define the Cpk and Z score formulas first. I see. Try this walkthrough on z score and process capability. The best place is to sign up here.

Z-scores are calculated using standard deviations, which you also say in that section and immediately following ones. Captured data,n 5. When cpk and ppk are close in value it represents a stable process, and when they are far apart it shows an unstable process. My question is, how far apart can they be where one can say if the process is stable or unstable.

I m involved in manufacturing of pharma products. Total number batches are 10 and Cpk of assay of batches is 0. Is it acceptable?? If not then what would be the imapct of sample size on Cpk?? Please reply. I appreciate that you continue share the six sigma information to me. We have some questions about six sigma calculation. When we talking about the capability of a process , we usually use cpk to show how well the process is. We want to know the capability of the process. So we sample 32x and we can calculate the cpk of the dimension from the 32x data.

But how about a attribute data? And if we can transfer yield to sigma level. Do we need to measure the process drift according to Motorola , the long term drift is 1. Or we just need to calculate the short term sigma level? At first pass I think you can examine the relationship between z scores and process capability here. Am I interpreting your question correctly? The final sentence also implies that Cpk can be higher than Cp, which is not true.

Cp has nothing to do with whether a process is centered. There is no sample mean in the equation! Cp is the ratio of the spec range over the sample std dev. Again, this is a good question. Stay tuned…. In the framework of process evaluation and analysis, CP and CPK are used as indicators of processes, but as I seen, they are more oriented towards each metric in particular than the process as a whole, for example, I have a human talent management process, that has three metrics, you can calculate the CP based on the specification limits of each metric, the standard deviations of those The data, but how would the roll-up or grouping of information to measure the capacity of the human talent management process like everything?

I only have the capacity to answer these kind of questions in the paid forum. This is a great question. Right now I only have the capacity to answer these kind of questions in the paid forum. Upper specification limit: 8. We are writing to request copyright permission for teachers and students to view the following link for completing Project 2 in Course 3 Advanced Concepts in Materials Joining :.

Also, no changes in or deletion of author attribution, trademark, legend, or copyright notice will be made, if permission is granted. James J. Atlanta, GA Phone , ext. The chart below shows how a control chart and histogram of the data might look. Most of us would want to have the second process with the higher Cpk, but is the quality of the process necessarily better? Unless you determine whether the process is in statistical control, you cannot fairly answer this question.

As it turns out, the data is exactly the same, but what has changed is the order in which the data was grouped in the samples. This caused the range of the subgroups and R-bar the average range to be different. In the second data set, the data was rearranged so that the data within the sample is similar. The sigma of the individuals does not change, but the estimated sigma, which is used in the control limits and Cpk calculations, changes between the two distributions.

With this example, determining if the process is in control before looking at Cpk pays off. Since the control chart in the second example, shown below, is not in statistical control, you cannot be sure that its Cpk is a good representation of process capability. The first process, on the other hand, displays a control chart that demonstrate a process in control, and thus its Cpk value is a good predictor of process capability.

I asked the below question of a software provider hoping he could explain why his software was giving me a Cpk greater then 1. Can any one out there help me better understand? It simply measures the spread. Technically, your software provider is right although not very helpful. The Cpk is an indicator of how centered your process is use Cp and Cpk together to evaluate this. The Cpk calculation assumes that the data is normally distributed.

From the sounds of it, your data may not be. A skewed distribution would throw off the range. Ensure measurement system analysis rules are followed. After reading materials for capability index, it was clear that value above 2 for Cp and CpK is great. 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.