Rational subgrouping is a data sampling technique used in control charting that determines the sensitivity, and therefore the usefulness, of the chart. Essentially, the control limits are determined by the variation within subgroups which is assumed to represent common cause or random variation only, and this establishes the chart's ability to gage variability between subgroups.
In order to ensure that the within-subgroup variation only comprises common causes, the observations within each subgroup should be as homogenous as possible, that is they should be collected under essentially the same conditions.
What Is a Rational Subgroup? By Donald J. Wheeler: - http://www.qualitydigest.com/oct97/html/spctool.html