The Pp and Ppk are generally lower as the Cp and Cpk values. When the value is higher the Cp and Cpk are calculated.
The Pp and Ppk is calculated when the subgroup is set 0. Label the data in subgroups to identify the factors that can cause that the process is not in control.Do a Gauge R&R and try to improve the measurement.The normally check is calculated on the overall data and does not use the subgroups! The Cp and Cpk assume that the data is according a normal distribution, but due use of subgroups less vulnerable for non normal overall data. What if the process is not approximately normally distributed The minimum sample size to estimate the Cp and Cpk is 30 samples. When subgroup size of 5 is selected then row 1 to 5 is for subgroup 1 and row 6 to 10 is for subgroup 2 etc. When calculating the STDEV within data is ordered in groups of 5 to filter out the drift between the different groups.īut it includes the short therm variation of the process. If in a production process after every hour 5 measurements are done, the subgroup size is 5. >1 there are subgroups and the Cp and Cpk are calculated with the estimated STDEV within. With a higher Cpk value the data set is better within tolerance.Ġ is no subgroups the overall STDEV will be used and the Pp and Ppk will be calculated.ġ is no subgroups the STDEV within will be estimated and the Cp and Cpk will be calculated. Violet the process is probably not in control the difference between Cpk and Ppk is bigger as 15%. Therefore the Cp is always lower of equal compared to the Cpk value. To get this quality (Cpk) the process needs to be tuned so that mean is moved to center of the tolerance borders. The Cp does not include the drift between the subgroups! If you want to included the drift use the Pp. The Cp indicates if a process is capable to produce within the specification limits, if the process is centered between the specification limits. For not production environment (testing, new process or setting) use the Pp and Ppk! The Cp and Cpk is used when a process is under statistical control. When this value is 0 the Pp and Ppk are calculated. The Cp and Cpk is calculated when the subgroup is set 1 or higher. The Cp Cpk Pp and Ppk are used together with Control charts in Statistical process control (SPC) To check if the production process is capable to produce within specification limits.
HOW TO CALCULATE SIX SIGMA 4.5 HOW TO CALCULATE PPM FULL VERSION
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