A ppk of 1 means that there is half of a bell curve between the center of the process and the nearest specification limit. This topic has 4 replies, 5 voices, and was last updated 16 years, 10 months ago by ron. Issues with cp, cpk, pp, ppk process capability indices. Cp cpk pp ppk process capability analysis in excel. Interpreting pp, ppk process performance index basically tries to verify if the sample that you have generated from the process is capable to meet customer ctqs requirements.
An internal or external process is considered to be six sigma if cp is equal to or greater than 2. Process capability indices datalyzerspectrum denmark. To ppk or not to ppk and here is where the danger lies in only reporting cpk and forgetting about ppk like its george michaels lesserknown bandmate no offense to whoever he may be. I wonder whether i have to use diff function to remove unit root or not. Calculate the ppk index and take the following actions. An important part of any spc implementation is the use of process capability indices.
It provides a comparison between the output of a process versus the process specifications. My next blog will describe an alternative approach to process capability reporting that not only addresses the above issues but also provides a statement that is. Process capability index cpk is the measure of process capability. A pp plot compares the empirical cumulative distribution function ecdf of a variable with a specified theoretical cumulative distribution function. It issues a buy on upward moving stocks, small cap stocks when the gravity trend crosses the momentum line forming an x. Interpret the key results for normal capability analysis. The natural tolerance of the process is computed as 6s.
The outputs of any process will vary, as discussed in the chapter on variation, and it is common for specification limits to be defined such that if the. The capability study is one of the main tools of six sigma. P pk min p pl,p pu for a capable process, these values should be equal to or greater than 1. Statistical process control basics 70 glen road, cranston, ri 02920 t. Larger the p pk, lesser is the variation between process output and specifications. Im not entirely sure what citation style youre using, but this convention is used in the mla citation style, as well as some other citation styles that i cant remember off the top of my head literaturese user aethelbald has suggested that. Over the years javascript has had its ups and downs. Construction and interpretation of pp plots a pp plot compares the empirical cumulative distribution function ecdf of a variable with a specified theoretical cumulative distribution function f. The x with double bar over it is the overall average. The conversion curve presented here can be a useful tool for interpreting.
Read, highlight, and take notes, across web, tablet, and phone. The difference between cp and pp, as well as between cpk and ppk, results from the method of calculating standard deviation. This is mainly due to the flux of poorly optimized scripts that are scattered around. From p pk value, one can drive that machine a is producing 99. It assumes the process mean is centered between the specification limits and essentially. Posted 06162014 840 views in reply to anum id first look at the plot of residuals vs the explanatory variabless to see if the residuals have any pattern not good fit or seem randomly distributed good fit. How well the data would fit within the spec limits usl, lsl ppk.
Cpk is a short term process index that numerically describes the within subgroup or potential capability ppk is long term indicator of a process assuming it was analyzed and stays in control. For pp and ppk calculations, the standard deviation used in the denominator is based on all of the data evaluated as one sample, without regard to any subgrouping. In both cases we want to try to verify if the process can meet to meet customer ctqs requirements. Its an option along with zscore and ppm when describing process baseline measurement in the measure phase or in the control phase. S chart for x group group summary statistics 1 4 7 10 16 19 22 25 28 31 34 37 40 0.
Process capability is the longterm performance level of the process after it has. Walther pp ppk pistol explained downloadable ebook h. 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. Six sigma isixsigma forums old forums general how to interpret cpk. Advanced dismounting of the slide 1 inner components of a walther pp slide dismounting of the manual safety removal of the firing pin different types of firing pins overall lengths of firing pins compared variations of the premid1970s manual safety. Introduction a process capability index is a numerical summary that compares the behavior of a product. If the system is centered on its target value, ppk should be used in conjunction with the pp. The ecdf, denoted by, is defined as the proportion of nonmissing observations less than or equal to, so that. Although spc control charts can reveal whether a process is stable, they do not indicate whether the process is capable of producing acceptable outputand whether it is performing to capability. Process capability index cpk formula with solved examples. When you have the total population and are testing the performance of a system to meet customer needs. The ppks magazine holds one additional round, in both calibers. Process performance indices pp, ppk measure the relationship of the process bell curve to the specification limits.
The quality engineer should continually refer to this principle when interpreting capability indexes. Process performance index ppk, cpk, pp what is six. There are two versions of normal probability plots. Pp and ppk indices is it statistically possible to have. Describe the changes in relationships that occur when longterm data are used. Ppk on javascript by peterpaul koch might just be the most thorough javascript book i have read. The concepts are explained here in such a way that anyone new to six sigma can understand the logic and use them with ease.
The conversion curve presented here can be a useful tool for interpreting cpk with its corresponding defect levels. Ppk can be determined by diving the z score by three. Capability cp and performance cpk charts illustrate a processs ability to meet specifications. Understanding process capability indices statistics and actuarial. If pp and ppk differ, then the process is not centered. Generally you use this when a process is under statistical control. Pp, ppk are more liberal where cp, cpk are more conservative. Varies alterations over the years and all the different special made models for use during ww2. We can see from the examples above that cpk only tells us part of the story, so the next time you examine process capability, consider both your cpk and. Process capability analysis for normal distributions cp and ppm interpretation the figure below shows control charts for a process. Process capability cp, cpk and process performance pp. In this article we will discuss the most common used indices cp, cpk, pp and ppk.
It measures how well the data might fit between the spec limits usl, lsl. However, spc also has a major role as a tool for the monitoring and control of manufacturing defects commonly known as attributes. 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. Its more precise than a histogram, which cant pick up subtle deviations, and doesnt suffer from too much or too little power, as do tests of normality. So the next question is which metric is best to report cpk or ppk. Process capability with minitab lean sigma corporation. Difference between ppk and ppks difference between.
Understanding process capability indices stefan steiner, bovas abraham and jock mackay institute for improvement of quality and productivity department of statistics and actuarial science university of waterloo waterloo, ontario n2l 3g1 1. Hoffschmidt tells you allmost everything you need to know about the walther pp and ppk. It doesnt care if the process is centered within the limits only if it would fit if it was centered. I have difficulty in solving unit root test by using both adf. Yes, different booksarticlespeople handle s and sigma differently or call them both s as you said. Cp historically, this is one of the first capability indexes used. Statistical process control is most usually associated with measured product characteristics. Process capability is a relatively simple statistical measure which provides an estimate on the level of process outputs which will be within permitted specification limits. Both cpk and ppk are the minimum of two process indices. The equations for cpk and ppk are shown in table 1.
Samples from the batch may need to be quite large to be. The index simply makes a direct comparison of the process natural tolerance to the. It shows how closely a process is able to produce the output to its overall specifications. A person may perform with minimum variation, but he can be away from his target towards one of the specification limits that. The ppks passed the import point test required in the u. The ecdf, denoted by f n x, is defined as the proportion of nonmissing observations less than or equal to x, so that f n x i in to construct a pp plot, the n nonmissing. When a process is in statistical control, ppk and cpk are approximately equal. Measuring your process capability symphony technologies.
Sigma levels a sigma level is nothing more than a zscore it measures the number of standard deviations of the process that can fit between the process average. Anatomy of a normal probability plot the analysis factor. For a normal distribution, the mean equals median, which is 50th percentile, and. If ppk is lower than your benchmark, consider ways to improve your process. If pp and ppk are approximately equal, then the process is centered between the specification limits. Could someone please tell me if it is statistically possible to have a ppk greater than pp. When to use it how to understand it example how to do it practical variations. 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. But the statistical measurements of cp, cpk, pp, and ppk may provide more. Ppk is a good measure when the process is not exactly centered in the middle of usl and lsl. It differs from process capability in that process performance only applies to a specific batch of material. Define, select, and calculate pp, ppk, cpm, and assess process performance. The quality toolbook process capability how to understand it.
Through the use of eight realworld example scripts, ppk walks us through javascript from the very beginning. It decides how consistent we are to our average performance. Ppk index comparison of shortterm and longterm capability indices mindpro leaditig to higher profits. Process capability cp, cpk and process performance pp, ppk. It helps us understand what our process capability is and tells us the probability that the process will produce defects. Interpret the key results for normal capability sixpack. Index defects dpu dpmo dpo final first time through put roll cp cpk ppk bench short term long term for the y variable and the x variable usi longitudinal instantaneous tools to help us guide our activity we want to understand the. A normal probability plot is extremely useful for testing normality assumptions. Compare ppk to a benchmark value that represents the minimum value that is acceptable for your process. Using our cmm we have measured the true position location of a hole in an a pillar. The two metrics starting with c measure short term capability. There are several capability indices cp, cpk, ppk, cpm, ncpk. What is the difference between the ppk values reported by statit and the cpk values.
1223 870 153 1067 403 391 1335 893 217 857 1276 1183 768 809 722 786 489 78 975 1433 355 169 462 212 1319 424 989 760 1286 432 227 827 611 649 637 904 408 1465 1117 378 881 1284 519 943 938 696