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Example of. Normal Capability Sixpack. Learn more about Minitab. An engine manufacturer uses a forging process to make piston rings. The quality engineers want to assess the process capability. They collect 25 subgroups of five piston rings and measure the diameters. The specification limits for piston ring diameter are 74.0 mm ± 0.05 mm.
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. Key Result: Ppk. For these process data, Ppk = 0.52. Because Ppk is less than 1.33, the overall capability of the process does not meet customer requirements.
Key Result: Cpk. For these process data, Cpk is 1.09. Because Cpk is less than 1.33, the between/within capability of the process does not meet customer requirements. The process data are too close to the lower specification limit. The process is not centered, so Cpk does not equal Cp (2.76).
Z.Bench for Overall Capability. Z.bench (overall) is the percentile on a standard normal distribution that translates the estimated probability of defects in the process to an upper tail probability. It is calculated based on overall process performance, using the overall standard deviation. The defects for the process fall on both sides of the ...
When to use an alternate analysis. If you want to evaluate a more complete set of capability measures, in addition to the major indices, use Normal Capability Analysis.. If you want to evaluate the assumptions for using nonnormal capability analysis, use Nonnormal Capability Sixpack.. If your process produces large variation between subgroups, such as a batch process, use Between/Within ...
Step 1: Check your data for problems. Your process should be stable and the process data should follow the nonnormal distribution that you selected for the analysis. The control charts and probability distribution plot allow you to evaluate whether these requirements are met.
Use an R chart to monitor the variation of a process and determine whether it is stable enough to perform capability analysis. The R chart indicates whether the subgroup-to-subgroup variation is in control. Red points indicate subgroups that fail at least one of the tests for special causes and are not in control.
Use the control charts in the capability sixpack output to determine whether the process is stable and in control. The data should follow a normal distribution. The process capability estimates for this analysis are based on the normal distribution. If the data are not normally distributed, the capability estimates will not be accurate for your ...
The within-subgroup variation is the variation of coating thickness within one roll. The between-subgroup variation is the variation between rolls that may occur during machine resets. The engineer performs between/within capability sixpack to quickly evaluate the main capability indices and assess the assumptions for the analysis.
Under Data are arranged as, select Single column.; In Single column, enter the column of numeric data that you want to analyze.; In Subgroup size, enter a column that identifies the subgroup for each measurement or enter a number that indicates the subgroup size. To perform a between/within analysis, you must have at least 2 observations in at least one subgroup.