Process capability and sigma level applicability in it. In 1991, the asqcaiag task force published the fundamental statistical process control reference manual, which shows the calculations for cpk as well as ppk. 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. I have gone through the web but all i found a confusing and more confusing theoretical implication of the two. Capability indices there are several statistics that can be used to measure the capability of a process. Cp and cpk measure how consistent you are to around your average performance. The requirement in industry is that the ppk value should exceed 1.
Understanding process capability indices statistics and actuarial. Cp and cpk consider the deviation mean within rational subgroups, while pp and ppk set the deviation based on studied data. I am trying to understand the mathematical difference between cpk and ppk used in statistical process control spc. When ppk stands for process performance index, cpk stands for process capability index. Ppk calculations use actual sigma sigma of the individuals, and shows how the system is actually running when compared to the specifications. 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. The only acceptable formula for cpk uses the estimated sigma.
Given that ppk uses the calculated sigma, it is no longer necessary to use the calculated sigma in cpk. Ppk is the minimum between ppku and ppkl usl upper specification limit lsl lower specification limit. The concept of process capability only holds meaning for processes that are in a state of statistical control. The only difference lies in the denominator for the upper and lower statistics. 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 main difference between pp and cp on one side and ppk and cpk on the other side is whether. That made the ppk s less comfortable to shoot for me. Ppk tends to be more realistic because it doesnt cut out real data, while cpk data gathering could inadvertently skew the data into looking like a process is capable but not addressing its performance. 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.
I first went to the qs9000 spc manual for guidance. Cp, cpk, pp, ppk, probability plotting, and six sigma forrest w. 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. You will also find out whether your values meet the requirements for calculating machine.
To illustrate the calculation of the estimated capability indices cpk and ppk we present a. The described process capability cp, cpk, pp, ppk issues and resolution approach to process performance reporting provides improved process understanding. The difference between the two is the way the standard deviation is calculated. Mar 19, 2009 as mentioned by sj, the cpk is a short term capability and ppk is the long erm capability. Resolving the process capability index issues of cp, cpk, pp, and ppk is very important for organizations. Feb 06, 2015 we use your linkedin profile and activity data to personalize ads and to show you more relevant ads. May 23, 2018 theres a lot of confusion about capability statistics, here is my take on them. I may be mistaken, but the manual seems to indicate that cpk is a measure of process capability after disregarding any data recorded during a. Cp and cpk are based on short term variability pp and. Cp is the simplest indicator of a process capability while cpk gives a better picture. Byjus online cpk calculator tool makes the calculation faster, and it calculates the performance of the process in a fraction of seconds. Regardless of the index that is used, capability studies can evaluate how good the process is, with respect to target values or specifications. Cp and cpk are important parameters in statistical process control.
Cpk is typically used while processing in the ideal conditions to identify if the process is capable of meeting the specifications. Ppk helps in verifying if the sample generated is capable to meet customer ctqs. Cp cpk pp ppk process capability analysis in excel. This video is from our blog series on process capability and it takes a look at two common process capability indexes cpk and ppk and. Ppk informs the user of how the process has performed in the past. Another way to look at the difference is that cpk is used for a subgroup of data, while ppk is used for the whole process.
Notice the difference between shortterm and longterm is an industry standard 1. The statistics assume that the population of data values is normally distributed. For pp and ppk calculations, the standard deviation used in the denominator is based on all of the data evaluated as one. For the love of physics walter lewin may 16, 2011 duration.
Cpk is more widely used than cp, since it takes into account the mean and the standard deviation in its calculation. The reason for this difference is illustrated in figure 6. Cp is also known as the process potential index while cpk is known as process capability index or process performance index. 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. What is the mathematical difference between cpk and ppk. You may download a pdf copy of this publication at this link. What is the difference and why it is important medical.
Use control charts to evaluate the stability of your process. Process capability cp, cpk and process performance pp, ppk. In other words, if cpk ppk, the process is likely in statistical control. In process improvement efforts, the process capability index or process capability ratio is a statistical measure of process capability. Cpk calculator is a free online tool that displays the process performance for the specified range.
Process capability cp, cpk and process performance pp. Cp is more a measure of potential capability and cpk actual capability cpk compares the process average to the specification target. Cp index does not take into consideration the placement of process with respect to the given limits or the. Minimum value from cpkl and cpku is the value of cpk by observing results of statistical analy sis one can be explain or determine future expected process capabilities ex. 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. Pdf process capability and capability index cp and cpk. The ppk has wraparound grips and the ppks has an exposed metal backstrap. I have only recently just like your postings ironically been trying to figure out the difference between cpk and ppk.
This resolution is easily accomplished with 30,000footlevel reporting. In the absence of any special or assignable causes of variation, a process will still have some inherent variability. So the key takeaway is that cpk is the potential of a process to meet a specification short term while ppk is how the process actually did long. The larger the ppk, the less the variation between process output and specifications. In this article you will learn how to calculate machine and process capability using excel. A distinction between product and process should be made at this point. 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. This index also takes into account how well the process is centered within the specification limits. When quantifying process capability, the primary difference between cp and cpk is. In the absence of any special or assignable causes of variation, a. Cpk informs the user about the future capability of a process. 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. Resolving process capability index issues for cp, cpk, pp, ppk.
When the average of the process approaches the target value, the gap between cpk and cp. Perhaps cpk, ppk, cpm, or some other index offers the best means of reporting the capability of her product or process. Cp, pp, cpk, and ppk are capability and performance indicators for your process. Sep 06, 2011 however, there are issues with traditional process capability indices reporting. When the process capability index shows how a process will do in the future, the process. Theres a lot of confusion about capability statistics, here is my take on them. The cpk is an index which tells you how centered your data is within your specification range. There is a direct correlation between cpk or ppk and parts per million defects because only cps or pps and cpks or ppks that are greater than 1. 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. They are calculated using the subgrouped data variation to estimate population variation. The issue of process capability became a little murky when, in 1991, the asqaiag task force introduced two process performance indices pp and ppk to go along with the process capability indices cp and cpk.
Process capability cp, cpk and process performance pp, ppk what is the difference. Process capability indices measure how much natural variation a process. Cpk is calculated using the within standard deviation, while ppk. For stable processes and normally distributed data, a cpk value. 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. This document describes the differences between cpk and ppk values. It is worth working with more conservative pp and ppk data in case it is unclear if the sample. Fro simplicity you could create a small table where in cpk0. 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. Pdf what is process capability, how to calculate and how to use during.
Unlike the cp, the cpk is measured using standard deviation, specification limits and mean. For example, the calculated cpk, which uses the withinsubgroup estimate of the process variability is 1. Issues with cp, cpk, pp, ppk process capability indices reporting. Most people have at least heard of cp or cpk, but few people really understand the concepts behind the calculations. Some organizations consider cp and cpk are a measure of shortterm capability, while pp and ppk are a measure of longterm capability. 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. Process capability by sanjaya kumar saxena the capability of a process is a statistical indicator of how well it is functioning, or, in other words, how successful it is at running within its specified limits. Aug 01, 2018 cp and cpk are important parameters in statistical process control. 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. Thus, the major difference between cpk and ppk is the way the process variation. The x with double bar over it is the overall average. Hilti specification for process performance and process capability 6. Cpk is calculated using the within standard deviation, while ppk uses the overall standard deviation.
It measures how well the data might fit between the spec limits usl, lsl. An internal or external process is considered to be six. Dear sandeep, as mentioned by sj, the cpk is a short term capability and ppk is the long erm capability. Cpk is what the ppk will be if the process is stable no change between subgroups mean. Ppm and cpk are six sigma quality management terms used in manufacturing. Cpk values are known as process capability measures. Control limits, specification limits and process capability. One difference between the two that made the decision for me was the design of the grip.
The difference between cpk and ppk indicates if the process is stable or in other words if there are special causes of variation which are influencing the average of. The process capability cpk, ppk and the machine capability cmk describe the ability to achieve a desired result. Cpk and ppk values will be distinctly different, perhaps by a very wide margin. When you have a sample, not the population, and are testing the potential capability of a process to meet customer needs cp cpk formulas use sigma estimator. Companies ascribing to a six sigma methodology work toward reducing defects to a low rate six standard deviations away from the mean or 99. If the system is centered on its target value, ppk should be used in conjunction with the pp index. The difference between cp and pp, as well as between cpk and ppk, results from the method of calculating standard deviation. It doesnt care if the process is centered within the limits only if it would fit if it was centered. Both ppk and cpk have very wide impacts on the performance strategy of organisations and they are very closely watched. Issues with cp, cpk, pp, ppk process capability indices. The equations for cpk and ppk are shown in table 1. However, there are issues with traditional process capability indices reporting.
Histograms are preferable since they also provide other useful process. Two tables below provide both shortterm and longterm conversions of sigma level zscore, cpk or ppk, and defect rate respectively. Capability statistics cpcpk vs ppppk and sigma level. We use your linkedin profile and activity data to personalize ads and to show you more relevant ads. That made the ppks less comfortable to shoot for me. Before we proceed to define cp and cpk, certain basics related to process control should be explained for better understanding.
The difference between cp and cpk and when they are used. Variability can be stated as either shortterm or longterm. The ppk has wraparound grips and the ppk s has an exposed metal backstrap. How well the data would fit within the spec limits usl, lsl cpk. Cp is what ppk will be if the process is stable and centered. 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.
Both cpk and ppk are the minimum of two process indices. What is the difference between the ppk values reported by statit and the cpk values. Ppk is an index of process performance which tells how well a system is meeting specifications. That added to the ppk having a shorter grip making it easier to conceal and it was an easy decision.
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