Overview for Normal Capability Analysis for Multiple Variables

Use Normal Capability Analysis for Multiple Variables to compare the capability of multiple process variables or multiple groups within one process variable, based on a normal distribution. Using this analysis, you can do the following:

Determine whether the process is capable of producing output that meets customer requirements.
Compare the overall capability of the process with its potential (within) capability to assess opportunity for improvement.

To perform the analysis, you must specify a lower or upper specification limit (or both) to define your process requirements. The analysis evaluates the spread of the process data in relation to the specification limits. When a process is capable, the process spread is smaller than the specification spread. The analysis can also indicate whether your process is centered and on target. In addition, the analysis estimates the proportion of product that does not meet specifications.

For example, a manufacturer uses two machines to produce connector pins. The specification range for the length of the pins is 13 to 25 mm. The manager uses normal capability analysis for multiple variables to compare the process capability of the two machines.

Note

This analysis includes transformation functions to transform nonnormal data to fit a normal distribution. You can also use this analysis to perform a between/within analysis for multiple variables.

Where to find this analysis

To perform normal capability analysis for multiple variables, choose Stat > Quality Tools > Capability Analysis > Multiple Variables (Normal).

When to use an alternate analysis

If you do not know whether your process data are in control or whether they can be evaluated using a normal distribution, use Normal Capability Sixpack to assess these assumptions before you use this analysis.
If your data are nonnormal and you want to evaluate process capability for each variable by fitting a nonnormal distribution, instead of transforming your data, use Nonnormal Capability Analysis for Multiple Variables.
If you have only a single process variable, without a grouping variable, use Normal Capability Analysis or Between/Within Capability Analysis.