What is important to know before we will explain the definitions of the indices is that the. The main difference between pp and cp on one side and ppk and cpk on the other side is. Instead this article is intended to explain the difference between the two and why the different calculations are important. Ppk uses the calculated standard deviation from individual data where s is. 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. 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. It will also attempt to explain the differences between process capability and. The process capability requirements given in section 4. 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.
Thus, the major difference between cpk and ppk is the way the process. Understanding process capability indices statistics and actuarial. Cpk and ppk are two commonly used measures of process capability how well your. In the short sense, cpk and ppk are about process capability and process performance, respectively. But the statistical measurements of cp, cpk, pp, and ppk may provide more. If cpk is significantly different than ppk, the process is not in statistical control. 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.
Cpk ppk monitoring and trending data of process batches. If cpk is approximately equal to ppk, the process is in statistical control. Cpk and ppk values will be distinctly different, perhaps by a very wide margin. If we look at the formulas for cpk and ppk for normal distribution process capability, we can see they are nearly identical. Process capability, cpk, is important because it indicates whether a process potentially can meet a specification.
You may download a pdf copy of this publication at this link. It measures how well the data might fit between the spec limits usl, lsl. What is the difference and why it is important medical. 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.
Are you setting requirements for cpk while you should be. 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. 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. Cp and cpk are quantitative expressions that personify the variability of your. 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. 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 a chronically unstable, yet predictable process. Cpkppk monitoring and trending data of process batches. Process capability example,how to calculate cp and cpk. In other words, if cpk ppk, the process is likely in statistical control. Process capability cp, cpk and process performance pp.
The real truth behind cpk and ppk capability and potential. Pdf what is process capability, how to calculate and how to use during. Cpk is the shortterm capability of a process, and ppk is the longterm. Cp and cpk measure how consistent you are to around your average performance. Measuring your process capability symphony technologies. To illustrate the calculation of the estimated capability indices cpk and ppk we present a. Process capability cp, cpk and process performance pp, ppk. It doesnt care if the process is centered within the limits only if it would fit if it was centered. Pdf process capability and capability index cp and cpk. For illustration, lets consider a data set where 5 measurements were taken every day for 10. The requirement in industry is that the ppk value should exceed 1. 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. Ppk values are another process performance measure. Ppk helps in verifying if the sample generated is capable to meet customer ctqs.
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