Difference between cpk ppk pdf

Here is where the difference between within and overall standard deviation is apparent. Because cp and cpk use estimated sigma, and pp and ppk use actual sigma, they have different uses. For illustration, lets consider a data set where 5 measurements were taken every day for 10 days. Cpkppk monitoring and trending data of process batche s.

Process capability and capability index cp and cpk. That made the ppks less comfortable to shoot for me. The difference between cp and cpk indicates if the process is producing in the middle of the tolerance. Hence the process performance index, ppk is given by the smaller of the two values above. 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. Cp is also known as the process potential index while cpk is known as process capability index or process performance index. In 1991, the asqcaiag task force published the fundamental statistical process control reference manual, which shows the calculations for cpk as well as ppk. Process capability has been defined by many scholars and practitioners. Pdf what is process capability, how to calculate and how to use during.

Measures of process capability process capability indices. 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. Unlike the cp, the cpk is measured using standard deviation, specification limits and mean. Cp index does not take into consideration the placement of process with respect to the given limits or the. Histograms are preferable since they also provide other useful process. Process capability analysis and process analytical technology. In the absence of any special or assignable causes of variation, a process will still have some inherent variability. This example is a twotailed distribution based on the upper and lower control limits diameter specification. 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. 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. If cpk is significantly different than ppk, the process is not in statistical control. 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. C p and c pk in a similar way as was described with p p and p pk.

Cpk informs the user about the future capability of a process. 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. It measures the performance of the process related to both dispersion and centeredness. Ppk informs the user of how the process has performed in the past. To further illustrate the differences and similarities between cpk and ppk we. What are the differences between cpk and cmk and how to. In 1991, asq aiag task force published the statistical process control reference. Process capability indices measure the degree to which your process produces output that meets the customers specification. 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 performance index, is a statistical tool to verify if the sample that have generated from the process is capable to meet the customer requirement.

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. The difference between cp and pp, as well as between cpk and ppk, results from the method of calculating standard deviation. But, standard deviation is measured from the control chart as this is a measure of process performance. 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 the absence of any special or assignable causes of variation, a. Total variability is used in the denominator instead of the short term. 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. 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. If the process is in control and the distribution is well within the specification limits. That added to the ppk having a shorter grip making it easier to conceal and it was an easy decision. Issues with cp, cpk, pp, ppk process capability indices. It is worth working with more conservative pp and ppk data in case it is unclear if the sample.

Cpk measures how well the process mean is centered within the spec limits, and what percentage of product will be within spec. Cp is the simplest indicator of a process capability while cpk gives a better picture. Differentiating between cpk and ppk values lets take the confusion out of cpk and ppk. Capability indices help us judge whether the process is capable of producing parts that are within specifications. In the short sense, cpk and ppk are about process capability and process performance. Thus, the major difference between cpk and ppk is the way the process variation. 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. Process capability cp, cpk and process performance pp, ppk. Most people have at least heard of cp or cpk, but few people really understand the concepts behind the calculations. One difference between the two that made the decision for me was the design of the grip. Cp vs cpk pp vs ppk practical use of statistics point.

Ppk vs cpk a good, clear explanation and how minitab. Sorry to tell, but definition of cpk and ppk is wrong, it. When the process capability index shows how a process will do in the future, the process. This will help set realistic goals and aid in avoiding unnecessary pressure to the bodies involved in the process. Ppk helps in verifying if the sample generated is capable to meet customer ctqs. 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. The difference between cp and cpk and when they are used. Therefore, cpk and ppk is taken as the lessor value of the two calculations.

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. Ppk ppk is an process capability index that assesses how close the process mean is from the specification limit. If we look at the formulas for cpk and ppk for normal distribution process capability, we can see they are nearly identical. Ppk, i know, is usually specified for shortterm study results, and uses the calculated standard deviation in its determination. How critical the measured process is to downstream processes. Potential within capability for normal capability analysis. Cp, pp, cpk, and ppk are capability and performance indicators for your process. It measures how well the data might fit between the spec limits usl, lsl.

It doesnt care if the process is centered within the limits only if it would fit if it was centered. 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. 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. When the average of the process approaches the target value, the gap between cpk and cp. 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. 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. Process capability cp, cpk and process performance pp. 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. What is the difference and why it is important medical. Cp and cpk are important parameters in statistical process control.

Process capability cp, cpk and process performance pp, ppk what is the difference. Cpk is more widely used than cp, since it takes into account the mean and the standard deviation in its calculation. Given that ppk uses the calculated sigma, it is no longer necessary to use the calculated sigma in cpk. The quality professional should understand why a process is not. 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. Points in the yellow area indicate processes that have ppk between the value specified and twice. Process capability refers to the performance of the process when it is operating under statistical control. 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. Good news there is an excel addin to do this for you. Just like control charts tell us about the stability of the process, capability analyses tell us about process capability. Capability indices and process performance indices, the relation between process capability and. There is a time and a place to use each of these numbers depending on the individual situation. Cp and cpk consider the deviation mean within rational subgroups, while pp and ppk set the deviation based on studied data. Instead this article is intended to explain the difference between the two and why the different calculations are important.

In any industry, understanding the true potential of a process is vital. Normally distributed and attempt with the rest of the process team to. This is a measure of process performance over a period of time. 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. 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. Pdf process capability and capability index cp and cpk. Cp cpk formulas versus pp ppk formulas whats the difference. The main difference between pp and cp on one side and ppk and cpk on the other side is whether. Cp and cpk can be used to measure a systems potential capabilities, while pp and ppk measure the systems actual performance.

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 ppk has wraparound grips and the ppks has an exposed metal backstrap. We need to pick minimum25 subgroups for measurement, preferably5 data points per subgroups. Cpk and ppk values will be distinctly different, perhaps by a very wide margin. Issues with cp, cpk, pp, ppk process capability indices reporting. In simple words, ppk is an index of process performance, which tells how well. For the love of physics walter lewin may 16, 2011 duration. 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 statistical process control reference manual published by aiag, capability falls under 2 categories. Process capability and capability index cp and cpk presentation pdf available. 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, cpl, cpu, and cpk are the four most common and timed tested measures of process capability. Aug 14, 2015 cpk cpk is an process capability index that assesses how close the process mean is from the specification limit. Instead of focusing on spec limits cpm focuses on how well the process mean corresponds. The only acceptable formula for cpk uses the estimated sigma. Automotive industrie requires a ppk better than 1,67 and cpk better than 1,33. If the process is in control and the distribution is well within the specification limits then the difference between the upper. Understanding process capability indices statistics and actuarial. Aug 01, 2018 cp and cpk are important parameters in statistical process control.

The real truth behind cpk and ppk capability and potential. Cp and cpk measure how consistent you are to around your average performance. You may download a pdf copy of this publication at this link. 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.

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 approximately equal to ppk, the process is in statistical control. 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. Ppk values are another process performance measure. In other words, if cpk ppk, the process is likely in statistical control. The demand must be realistic and it should be certain that.

Cpk cpk is an process capability index that assesses how close the process mean is from the specification limit. Before we proceed to define cp and cpk, certain basics related to process control should be explained for better understanding. The horizontal axis of the goal plot is the difference between the process mean and the target. The reason for the difference in calculated cp and cpk values is because of the standard deviation value that was used in the calculations. Learn the difference between cp cpk and pp ppk formulas and the different methods for calculating sigma estimator.