Sorry to tell, but definition of cpk and ppk is wrong, it. 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. The difference between the two indices is the way the process standard deviation s is calculated. Cp and cpk consider the deviation mean within rational subgroups, while pp and ppk set the deviation based on studied data. 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. The reason for the difference in calculated cp and cpk values is because of the standard deviation value that was used in the calculations. Pdf process capability and capability index cp and cpk. Cpk is more widely used than cp, since it takes into account the mean and the standard deviation in its calculation. It is worth working with more conservative pp and ppk data in case it is unclear if the sample. C p and c pk in a similar way as was described with p p and p pk. In other words, if cpk ppk, the process is likely in statistical control. The real truth behind cpk and ppk capability and potential.
Caution the cpk index represents only one side of the process curve, and does not measure how the process performs on the other side of. Cp, cpl, cpu, and cpk are the four most common and timed tested measures of process capability. 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. Process capability cp, cpk and process performance pp, ppk what is the difference. When the process capability index shows how a process will do in the future, the process. One difference between the two that made the decision for me was the design of the grip. The only acceptable formula for cpk uses the estimated sigma.
Because cp and cpk use estimated sigma, and pp and ppk use actual sigma, they have different uses. 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. How critical the measured process is to downstream processes. Cpk informs the user about the future capability of a process. If we look at the formulas for cpk and ppk for normal distribution process capability, we can see they are nearly identical. That added to the ppk having a shorter grip making it easier to conceal and it was an easy decision. Process capability cp, cpk and process performance pp. The difference between cp and pp, as well as between cpk and ppk, results from the method of calculating standard deviation. The difference between ppk and cpk represents the improvement in process capability that you could expect if shifts and drifts in the process were eliminated.
That made the ppks less comfortable to shoot for me. Ppk values are another process performance measure. Ppk helps in verifying if the sample generated is capable to meet customer ctqs. Here is where the difference between within and overall standard deviation is apparent. Differentiating between cpk and ppk values lets take the confusion out of cpk and ppk.
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. Sep 06, 2011 from the output of figures 1 and 2, we noticed a very different response for cp and cpk, while there was not much difference between pp and ppk. In the short sense, cpk and ppk are about process capability and process performance. Measures of process capability process capability indices. But, standard deviation is measured from the control chart as this is a measure of process performance. Process performance index, is a statistical tool to verify if the sample that have generated from the process is capable to meet the customer requirement. Process capability refers to the performance of the process when it is operating under statistical control. Cp is the simplest indicator of a process capability while cpk gives a better picture. In simple words, ppk is an index of process performance, which tells how well.
Ppk ppk is an process capability index that assesses how close the process mean is from the specification limit. Therefore, cpk and ppk is taken as the lessor value of the two calculations. Aug 14, 2015 cpk cpk is an process capability index that assesses how close the process mean is from the specification limit. In this study, at the first the process capability indices are presented then machine capability indices are discussed finally, process performance indices pp, ppk, and difference between cpk and. It measures the performance of the process related to both dispersion and centeredness. Cp and cpk can be used to measure a systems potential capabilities, while pp and ppk measure the systems actual performance. Cp, pp, cpk, and ppk are capability and performance indicators for your process. The ppk has wraparound grips and the ppks has an exposed metal backstrap. Points in the yellow area indicate processes that have ppk between the value specified and twice. It doesnt care if the process is centered within the limits only if it would fit if it was centered. 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.
Normally distributed and attempt with the rest of the process team to. 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. To further illustrate the differences and similarities between cpk and ppk we. This article is not how to calculate cpk or ppk, as there are several resources in print or on the internet to figure that out. The difference between cp and cpk indicates if the process is producing in the middle of the tolerance. 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. On page 59, longterm capability is denoted as cpk and uses an estimated process standard deviation while shortterm capability is denoted as ppk using the calculated standard deviation.
Process capability and capability index cp and cpk presentation pdf available. Im almost embarrassed to ask the question, but ive not been able to find a good, clear explanation of the real difference between the ppk and cpk indices. 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. 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. For illustration, lets consider a data set where 5 measurements were taken every day for 10 days.
If cpk is approximately equal to ppk, the process is in statistical control. Process capability has been defined by many scholars and practitioners. Whenever there is a significant difference between cpk and ppk for the same data, check the control chart for out of control conditions and consider the ppk value to be more accurate. 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. This is a measure of process performance over a period of time. Cp and cpk are important parameters in statistical process control. Just like control charts tell us about the stability of the process, capability analyses tell us about process capability. From the output of figures 1 and 2, we noticed a very different response for cp and cpk, while there was not much difference between pp and ppk. What is the difference and why it is important medical. This will help set realistic goals and aid in avoiding unnecessary pressure to the bodies involved in the process. Issues with cp, cpk, pp, ppk process capability indices reporting. Ppk informs the user of how the process has performed in the past. Ppk vs cpk a good, clear explanation and how minitab. Histograms are preferable since they also provide other useful process.
Before we proceed to define cp and cpk, certain basics related to process control should be explained for better understanding. 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. You may download a pdf copy of this publication at this link. Cpkppk monitoring and trending data of process batche s. Process capability and capability index cp and cpk.
The demand must be realistic and it should be certain that. Automotive industrie requires a ppk better than 1,67 and cpk better than 1,33. Caution the cpk index represents only one side of the process curve, and does not measure how the process performs on the other side of the process curve. Cp index does not take into consideration the placement of process with respect to the given limits or the. In the statistical process control reference manual published by aiag, capability falls under 2 categories. Process capability analysis and process analytical technology. Thus, the major difference between cpk and ppk is the way the process variation. There is a time and a place to use each of these numbers depending on the individual situation.
If the process is in control and the distribution is well within the specification limits then the difference between the upper. Cpk cpk is an process capability index that assesses how close the process mean is from the specification limit. We need to pick minimum25 subgroups for measurement, preferably5 data points per subgroups. 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.
Instead of focusing on spec limits cpm focuses on how well the process mean corresponds. Aug 01, 2018 cp and cpk are important parameters in statistical process control. Instead this article is intended to explain the difference between the two and why the different calculations are important. Ppk, i know, is usually specified for shortterm study results, and uses the calculated standard deviation in its determination. The horizontal axis of the goal plot is the difference between the process mean and the target. What are the differences between cpk and cmk and how to. Understanding process capability indices statistics and actuarial. Cp and cpk measure how consistent you are to around your average performance. Potential within capability for normal capability analysis. The quality professional should understand why a process is not. In the absence of any special or assignable causes of variation, a.
Learn the difference between cp cpk and pp ppk formulas and the different methods for calculating sigma estimator. Cp vs cpk pp vs ppk practical use of statistics point. Given that ppk uses the calculated sigma, it is no longer necessary to use the calculated sigma in cpk. Cpk and ppk values will be distinctly different, perhaps by a very wide margin. Hence the process performance index, ppk is given by the smaller of the two values above. Cp is also known as the process potential index while cpk is known as process capability index or process performance index.
The main difference between pp and cp on one side and ppk and cpk on the other side is whether. For the love of physics walter lewin may 16, 2011 duration. Unlike the cp, the cpk is measured using standard deviation, specification limits and mean. Process capability indices measure the degree to which your process produces output that meets the customers specification.
Good news there is an excel addin to do this for you. The difference between cp and cpk and when they are used. Cpk measures how well the process mean is centered within the spec limits, and what percentage of product will be within spec. This example is a twotailed distribution based on the upper and lower control limits diameter specification. When the average of the process approaches the target value, the gap between cpk and cp. If the process is in control and the distribution is well within the specification limits then the difference between the upper specification u and then mean or the difference between the lower specification l and the.
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. If cpk is significantly different than ppk, the process is not in statistical control. In any industry, understanding the true potential of a process is vital. Cp cpk formulas versus pp ppk formulas whats the difference. It measures how well the data might fit between the spec limits usl, lsl. Total variability is used in the denominator instead of the short term. Pdf what is process capability, how to calculate and how to use during. 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 even if. If the process is in control and the distribution is well within the specification limits.
Most people have at least heard of cp or cpk, but few people really understand the concepts behind the calculations. 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 even if control limits are not properly set. Capability indices help us judge whether the process is capable of producing parts that are within specifications. Capability indices and process performance indices, the relation between process capability and. Process capability cp, cpk and process performance pp, ppk. In the absence of any special or assignable causes of variation, a process will still have some inherent variability. 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. The goal of capability analysis is to ensure that a process is capable of meeting customer specifications, and we use capability statistics such as cpk and ppk to make that assessment. In 1991, the asqcaiag task force published the fundamental statistical process control reference manual, which shows the calculations for cpk as well as ppk. Issues with cp, cpk, pp, ppk process capability indices. In 1991, asq aiag task force published the statistical process control reference.