Pdf process capability and capability index cp and cpk. Apr 26, 2017 the technical difference is that the 6 sigma used for the cp calculation or the 3 sigma used for the cpk calculation comes from the estimate of sigma based on the average range, and the 6 sigma used for pp calculation or 3 sigma used for the ppk calculation comes from the estimate of sigma based on using all the data and the classical formula for the standard deviation. Use control charts to evaluate the stability of your process. The reason for this difference is illustrated in figure 6. Statistics and statistical techniques are a vital key to any world class organization however, statistics can be intimidating to many employees and managers. We use your linkedin profile and activity data to personalize ads and to show you more relevant ads. The only thing tying these two things together is the entirely arbitrary name of pvalue. I am trying to understand the mathematical difference between cpk and ppk used in statistical process control spc.
It measures how well the data might fit between the spec limits usl, lsl. Issues with cp, cpk, pp, ppk process capability indices reporting. Ppk is a process capability index that uses long term data. An important part of any spc implementation is the use of process capability indices. Both cpk and ppk are the minimum of two process indices. An internal or external process is considered to be six. The difference between cp and pp, as well as between cpk and ppk, results from. Total variability is used in the denominator instead of the short term.
Perhaps cpk, ppk, cpm, or some other index offers the best means of reporting the capability of her product or process. It is worth working with more conservative pp and ppk data in case it is unclear if the sample. The ppk calculation is identical to the cpk calculation, except that the sigma standard deviation value is based on a much larger sample size taken over a longer period of time i. For the love of physics walter lewin may 16, 2011 duration. That added to the ppk having a shorter grip making it easier to conceal and it was an easy decision. Ppm and cpk are six sigma quality management terms used in manufacturing. For pp and ppk calculations, the standard deviation used in the denominator is based on all of the data evaluated as one. If cpk is approximately equal to ppk, the process is in statistical control if cpk is significantly different that ppk, the process is not in statistical control so, when you looked at the supplier chart and noticed a big difference between cpk and ppk, you were given a key piece of information. What is the difference between the ppk values reported by statit and the cpk values. Process capability cp, cpk and process performance pp. You will also find out whether your values meet the requirements for calculating machine. Cpk is more widely used than cp, since it takes into account the mean and the standard deviation in its calculation. Cpk says, this is how my process is capable of running in the future. Cpk informs the user about the future capability of a process.
Process capability and sigma level applicability in it. I have gone through the web but all i found a confusing and more confusing theoretical implication of the two. Review of the development in process capability analysis. Indices divide a specification width between specification limits by a process width between control limits. When the process capability procedure in statit is performed based on this data, there are significant differences between the estimates of pp and cp and, analogously, ppk and cpk. The technical difference is that the 6 sigma used for the cp calculation or the 3 sigma used for the cpk calculation comes from the estimate of sigma based on the average range, and the 6 sigma used for pp calculation or 3 sigma used for the ppk calculation comes from the estimate of sigma based on using all the data and the classical formula for the standard deviation. Cpk mincpl,cpu indices are computed under the assumption that data values are independent samples from a normal population with constant mean and variance. Cpk is used far more often than ppk for expressing process capability. The indices pp and cp are not recommended for reporting purposes, since the information they provide to supplement cpk and ppk is also easily obtained from a histogram of the data. In 1991, the asqcaiag task force published the fundamental statistical process control reference manual, which shows the calculations for cpk as well as ppk.
Some organizations consider cp and cpk are a measure of shortterm capability, while pp and ppk are a measure of longterm capability. It doesnt care if the process is centered within the limits only if it would fit if it was centered. Some people like to use sigma value minitabtm reports this as zbench, other like to use cpk, ppk. Regardless of the index that is used, capability studies can evaluate how good the process is, with respect to target values or specifications. Z value, cp, cpk, pp and ppk can be quite confusing to understand and a task in itself to remember these concepts for six sigma green belt project leads and other project or process managers who want to understand their current and potential.
Apr 06, 2016 six sigma statistics with excel and minitab analysis of variance issa bassdownload from 4shared. Frequentlyasked questions about capability data collection. Under this, there is a term known as process improvement, which involves a series of actions to identify, analyze and improve a particular business process. Byjus online cpk calculator tool makes the calculation faster, and it calculates the performance of the process in a fraction of seconds. For example, the calculated cpk, which uses the withinsubgroup estimate of the process variability is 1. Feb 06, 2015 we use your linkedin profile and activity data to personalize ads and to show you more relevant ads. Please note that the difference between cp and cpk is an indicator of how far the average of the process is from the target specification. There is quite some confusion about the use of these indices. Dear sandeep, as mentioned by sj, the cpk is a short term capability and ppk is the long erm capability. Pp, and ppk are based on actual performance regardless of how you have sub grouped your data, and use the normal standard deviation calculation of all data n. Lindley and nearly everyone else admits this is not a real paradox. Cpk values are known as process capability measures.
Answering a question today caused me to look up how minitab calculates cp and pp when you have nonnormal distributions i was surprised. Fro simplicity you could create a small table where in cpk0. The technical difference is that the 6 sigma used for the cp calculation or the 3 sigma used for the cpk calculation comes from the estimate of sigma based on the average range, and the 6 sigma used for pp calculation or 3 sigma used for the ppk calculation comes from the estimate of sigma based on using all the data and the classical formula for. Process capability with cp, cpk, pp, ppk and z score unnap. Most people have at least heard of cp or cpk, but few people really understand the concepts behind the calculations. However, there are issues with traditional process capability indices reporting. The ppk has wraparound grips and the ppks has an exposed metal backstrap. Cp and cpk consider the deviation mean within rational subgroups, while pp and ppk set the deviation based on studied data. Thus, the major difference between cpk and ppk is the way the process variation. Six sigma process performance is reported in terms of sigma. Automotive industrie requires a ppk better than 1,67 and cpk better than 1,33. Process capability with cp, cpk, pp, ppk and z score by admin in six sigma on february 24, 2016.
What is the mathematical difference between cpk and ppk. Ppk is the minimum between ppku and ppkl usl upper specification limit lsl lower specification limit. When the process capability index shows how a process will do in the future, the process. This document describes the differences between cpk and ppk values. But the statistical measurements of cp, cpk, pp, and ppk may provide more insight into the. Six sigma statistics with excel and minitab pdf386 cpk,ppk. Cpk calculator is a free online tool that displays the process performance for the specified range. Process capability attribute data is alwayslongterm in the shifted condition since it. Pp and ppk index formula data analysis free online math. If you are using cpk and ppk you can easily tr anslate that into a z score or sigma level by dividing by 3. Cpk, ppk and nonnormal distributions smarter solutions.
Under this, there is a term known as process improvement, which involves a series of actions to identify, analyze and improve a. Just like control charts tell us about the stability of the process, capability analyses tell us about process capability. For illustration, lets consider a data set where 5 measurements were taken every day for 10 days. How can i improve cpk value, when it is less than 1. Download six sigma statistics with excel and minitab rar free. What is the difference in the formulas for cpk and ppk. The cpk and ppk measures will be very close when the process remains in a state of consistent statistical control, as both the actual sigma and estimated sigma will be similar.
The difference between cp and cpk and when they are used. Histograms are preferable since they also provide other useful process. Cp, pp, cpk, and ppk are capability and performance indicators for your process. When the average of the process approaches the target value, the gap between cpk and cp. The difference between cp and pp, as well as between cpk and ppk, results from the method of calculating standard deviation.
Understanding process capability indices statistics and actuarial. Ppk says, this is how my process was running when i collected my data. Process capability and capability index cp and cpk. Clearly, quality cannot be inspected into a product or process, therefore, the net steps will. Process capability cp, cpk and process performance pp, ppk. Cp, cpk, pp, ppk, probability plotting, and six sigma forrest w. The x with double bar over it is the overall average.
As the graph on the left side shows, there is not a lot of shift and drift between subgroups compared to the variation within the subgroups themselves. My next blog will provide an improved process capability reporting methodology that is easier to interpret and can be used throughout an organization when reporting process capabilityperformance. Hide answers a cp or pp is potential capability b cpk or. If youre a manager, supervisor or team leader in a business organization, there is a need for you to learn about od, or organizational development. They do not calculate cp and cpk for nonnormal distributed data, which is probably a good thing because of the small sample issues with normality. They are calculated using the subgrouped data variation to estimate population variation. The main difference between pp and cp on one side and ppk and cpk on the other side is whether we use complete. Cpk ppk monitoring and trending data of process batche s. Data 6 pp input range cell links statistics calculations c4d yield count below lsl count above usl sum protection email for questions and bugs cp cpk sigma shortterm data2 sub1 sigma shortterm data1. That made the ppks less comfortable to shoot for me. Sep 06, 2011 however, there are issues with traditional process capability indices reporting. Companies ascribing to a six sigma methodology work toward reducing defects to a low rate six standard deviations away from the mean or 99. Cp indicates how well a process is capable to produce consecutive products within the required variation. Cpk cpk is an process capability index that assesses how close the process.
Ppk is a short term analysis mostly done with 50 or 100 samples measured ppk is process potential capability cpk is long term analysis and is continous process capability. Given that ppk uses the calculated sigma, it is no longer necessary to use the calculated sigma in cpk. Ppk informs the user of how the process has performed in the past. Cpk is a measure that tells how close an organisation is in achieving the target and how much consistent an organisation is with regard to average performance. One difference between the two that made the decision for me was the design of the grip. Is it possible to have a ppk value of 10 and a pp number of 5. Although statistical training is widely available, if the knowledge is not used, it disappears quickly. There are several capability indices cp, cpk, ppk, cpm, ncpk. The difference between cp and cpk shows that as well as being able to indicate the capability of the process to produce within specification, in cpk, there is an indication of the ability of the process to conform to the target value. Issues with cp, cpk, pp, ppk process capability indices. Process capability cp, cpk and process performance pp, ppk what is the difference. Cpk, ppk and process drift due to tool wear example atch 1 atch 3atch 2 sample spread the same in all 3 samples ppk cpk bybatch cpk ppmppk ppmcpk ppmby batch cpk sigma 112 t cp 2.
The process capability cpk, ppk and the machine capability cmk describe the ability to achieve a desired result. The main difference between pp and cp on one side and ppk and cpk on the other side is whether we use complete set of data for calculation pp and ppk where we calculate real performance of the system, or we use sample preproduction, batch, logical subgroups where we calculate capability of the process. You may download a pdf copy of this publication at this link. Sorry to tell, but definition of cpk and ppk is wrong, it. In this article we will discuss the most common used indices cp, cpk, pp and ppk. Mar 19, 2009 as mentioned by sj, the cpk is a short term capability and ppk is the long erm capability. Differentiating between cpk and ppk values lets take the confusion out of cpk and ppk. When ppk stands for process performance index, cpk stands for process. In other words, its a comparison of a cdf with a ratio of a pdf pmf at two values. Unlike the cp, the cpk is measured using standard deviation, specification limits and mean. The only acceptable formula for cpk uses the estimated sigma.
If cp and cpk are much greater than pp or ppk 33% greater, your process may not be stable enough to conduct a capability analysis. The ppk index is calculated in a similar way as the cpk index and needs no further explanation. The basis of the cpk calculation is sigma hat or estimated standard deviation which is calculated by taking the average of the subgroup ranges rbar and dividing it by a factor. To illustrate the calculation of the estimated capability indices cpk and ppk we present a. Ppk helps in verifying if the sample generated is capable to meet customer ctqs. The equations for cpk and ppk are shown in table 1. In this article you will learn how to calculate machine and process capability using excel. The practical difference between ppk and cpk ymcdn. Cp cpk pp ppk process capability analysis in excel. If the process is stable and in control the estimate of pp is similar to the estimate of cp. Six sigma statistics with excel and minitab analysis of variance issa bassdownload from 4shared.
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