Overview for Between/Within Capability Analysis

Use Between/Within Capability Analysis to evaluate the capability of your process based on a normal distribution when your process naturally produces systemic variation between subgroups, such as a batch process. 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 between/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, it estimates the proportion of product that does not meet specifications.

For example, an engineer wants to assess the process capability of a paper coating process considering both within-subgroup variation and between-subgroup variation. 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 a between/within capability analysis to evaluate how well the coating thickness meets the customer requirements of 50 ± 3 microns.