Difference between cpk ppk pdf file

Pdf what is process capability, how to calculate and how to use during. Ppk helps in verifying if the sample generated is capable to meet customer ctqs. For illustration, lets consider a data set where 5 measurements were taken every day for 10. In the short sense, cpk and ppk are about process capability and process performance, respectively.

Cpk calculates the standard deviation of each subgroup and pools the results, whereas ppk calculates the standard deviation from all data as one continuous matrix. The requirement in industry is that the ppk value should exceed 1. Instead this article is intended to explain the difference between the two and why the different calculations are important. If cpk is significantly different than ppk, the process is not in statistical control. Ppk is calculated the same way as cpk, although it always denotes the total variation and doesnt require the process to be in control but it must still be normally distributed see montgomery. 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. In other words, if cpk ppk, the process is likely in statistical control. The real truth behind cpk and ppk capability and potential. The process capability requirements given in section 4. If we look at the formulas for cpk and ppk for normal distribution process capability, we can see they are nearly identical.

Process capability example,how to calculate cp and cpk. Pdf process capability and capability index cp and cpk. Cpk and ppk are two commonly used measures of process capability how well your. 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. To illustrate the calculation of the estimated capability indices cpk and ppk we present a. 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. What is important to know before we will explain the definitions of the indices is that the. Process capability cp, cpk and process performance pp, ppk. 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. Ppk uses the calculated standard deviation from individual data where s is. When the process capability index shows how a process will do in the future, the process. 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. What is the difference and why it is important medical. Process capability cp, cpk and process performance pp.

It measures how well the data might fit between the spec limits usl, lsl. 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. Measuring your process capability symphony technologies. Process capability, cpk, is important because it indicates whether a process potentially can meet a specification. Cpkppk monitoring and trending data of process batches. Cp and cpk measure how consistent you are to around your average performance. Thus, the major difference between cpk and ppk is the way the process. 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. But the statistical measurements of cp, cpk, pp, and ppk may provide more. Cp and cpk are quantitative expressions that personify the variability of your. If cpk is approximately equal to ppk, the process is in statistical control. Cpk ppk monitoring and trending data of process batche s.

Understanding process capability indices statistics and actuarial. 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. What is a chronically unstable, yet predictable process. 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. 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. Cpk ppk monitoring and trending data of process batches.