The Capability Analysis option measures the conformance of a process to given specification limits. By default, capability is calculated based on a normal distribution. However, if you fit a distribution (continuous or discrete), then the capability report is produced based on that fit. To use a fit distribution, select Spec Limits or set the Spec Limits column property. When you select the Capability Analysis option for a variable, the Capability Analysis window appears. Use the window to enter specification limits, distribution type, and information about sigma.

Note: To save the specification limits to the data table as a column property, select Save > Spec Limits. When you repeat the capability analysis, the saved specification limits are automatically retrieved.

<Distribution type>

By default, the normal distribution is assumed when calculating the capability statistics and the percent out of the specification limits. To perform a capability analysis on non-normal distributions, see the description of Spec Limits under Capability Analysis.

Note: There is a preference for Capability called Ppk Capability Labeling that labels the long-term capability output with Ppk labels. Open the Preference window (File > Preferences), then select Platforms > Distribution to see this preference.

Sigma Quality is frequently used in Six Sigma methods, and is also referred to as the process sigma. See Capability Analysis.

Shows the values (represented by Index) of the Benchmark Z statistics. According to the AIAG Statistical Process Control manual, Z represents the number of standard deviation units from the process average to a value of interest such as an engineering specification. When used in capability assessment, Z USL is the distance to the upper specification limit and Z LSL is the distance to the lower specification limit. See Capability Analysis.