Rational Subgrouping

Rational Subgrouping– Rational Subgrouping is a method in which data is organized into subgroups for process control charts. Rational subgroups for process control charts involve the use of process and product knowledge and judgment, but there are a few basic principles that should be followed.

• Usually, the subgroups should be chosen so that within subgroup variation is minimized. For example, keep in mind that for an Average and Range chart, it is the within subgroup variation that determines how sensitive the chart will be since the average Range (R-bar) is used in the calculation of the Averages chart control limits.
• The selection of rational subgroups should be consistent with the structure of the data from the process.
• The selection of rational subgroups should allow for quick identification of potential corrective actions once an out-of-control condition is identified.

For instance, a process might require the filing of one-gallon cans of an expensive liquid using a machine with three filling heads. The operator has taken some initial data on the process and is trying to decide how to organize it into a process control chart. The initial data is given below. The characteristic being measured is filled volume (in gallons).

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