Process Capability
Process capability analysis measures how well a process is performing compared to given specification limits. A good process is one that is stable and consistently produces product that is well within specification limits. A capability index is a measure that relates process performance, summarized by process centering and variability, to specification limits.
Specification Limits
If a column contains a Spec Limits column property and the Create Process Capability option on the launch window is selected, a Process Capability report is automatically created. This report is based on the normal distribution, unless the column also contains a Distribution column property. If the column contains a Distribution column property, the Process Capability report is based on the distribution specified in the column property.
Tip: To add specification limits to several columns at once, see Manage Spec Limits Utility in Quality and Process Methods.
If a column does not contain specification limits, select Process Capability from the red triangle next to the name of the analysis variable and set specification limits in the Process Capability Analysis window.
To save specification limits from a report to the data table as a column property, select Save Spec Limits as Column Properties from the Process Capability red triangle. When you repeat the process capability analysis, the saved specification limits are automatically retrieved.
Process Capability Analysis Window
The Process Capability Analysis window appears when you select the Process Capability option from the red triangle next to the name of the analysis variable.
Use the Process Capability Analysis window to specify options for the capability analysis, including specification limits, the underlying distribution for the analysis, and the estimation method for sigma. Process capability requires you to choose how to estimate sigma, the within-group (short-term) variation. Different suboptions appear depending on which process capability option you choose.
Figure 3.12 Process Capability Analysis Window
Enter Spec Limits
Specifies the Lower Spec Limit, the Target, and the Upper Spec Limit for the process capability analysis. At least one of these must be a nonmissing value. If you select the Show Limits option, the specification limits appear on the histogram in the Distribution platform report.
Process Capability Options
Depending on which option you choose, different additional options appear. Choose one of the following options:
Subgroup Size = 1
Sets the subgroup size to 1 and provides additional Moving Range options. See Moving Range Options in Quality and Process Methods.
Use Subgroup ID Column
Enables you to select a subgroup ID column and provides additional Subgrouping and Moving Range options. See Process Subgrouping and Moving Range Options in Quality and Process Methods.
Use Constant Subgroup Size
Enables you to set a constant subgroup size and provides additional Subgrouping and Moving Range options. See Process Subgrouping and Moving Range Options in Quality and Process Methods.
Use Historical Sigma
Assigns a historically accepted value for sigma. See Historical Information in Quality and Process Methods.
Use Nonnormal Distribution
Enables you to select a nonnormal distribution and provides additional Nonnormal Distribution Options. See Distribution Options in Quality and Process Methods.
Show Within Capability
(Available only when Subgroup Size = 1 or Use Nonnormal Distribution is selected in Process Capability Options.) Specifies if estimates of within sigma are shown in the report.
Specify Alpha Level
Specifies the significance level for confidence limits.
Process Capability Analysis Report
After you click OK in the Process Capability Analysis window, a Process Capability report appears that contains a capability report for the selected variable. For more information about this report, see Individual Detail Reports in Quality and Process Methods.
• For statistical details, see Statistical Details for the Process Capability Platform in Quality and Process Methods.
• For an example, see Example of Process Capability.
• For the Process Capability platform, see Process Capability in Quality and Process Methods.
Note: You can set preferences for many of the options in the Process Capability report in Distribution at File > Preferences > Platforms > Process Capability.