Rational sub grouping is the process of organizing the data into groups which were produced basically under the same conditions. In other terms, rational subgroups are samples in which we produce all of the items under conditions that will only occur in the presence of random effects (which can then be held responsible for the observed variations). 

The data is supposed to be a “snapshot” of the process over a very small amount of time, and the sequence that we set them in will depict how these snapshots will vary over some specified time. 

Rational subgroups help in the estimation process of the short term variations. Thus, Rational subgrouping is the basis for operating control charts in a successful manner. These variations later help us predict the long term variations and their control limits, depending o the type of causes for the variation (special or common).

Rational subgroups indicate the common cause variation in the process and also the data collection method. For example, a plastic molding machine produces 200 bottles per hour. Supervisor randomly selects 3 bottles every hour and measures the weight in grams. The four bottles are one subgroup.

Types of subgroups

Within subgroup: variation existed within subgroups, also known as inherent variation.

Between subgroup: variation between the subgroups due to special causes in the process.

In statistics, control charts plotting for mean and range for each subgroup. Then statistics compared to the control limits. The control limits are computed using within the variation of subgroups. Thus, it is important to select the subgroups that have only inherent variation. The effective way to improve the process quality by maintaining the minimum variation within the subgroups and also between the subgroups.

Rational subgroup properties

  • The subgroups are all formed from the observations that have been taken in a sequence that has been time-ordered.
  • The observations within the subgroups are autonomous, which means that no observation influences, or results from, another.
  • The observations within a subgroup come from a lone steady process. Otherwise, the other special causes that occur at high frequency within the subgroup which results in high variation between the subgroups.
  • Select the subgroups in a way that makes each subgroup as homogeneous as possible. This provides the maximum opportunity for estimating expected variation from one subgroup to another.

How do we know the need of Rational Subgroups?

  • Special cause variation in the process may divulge the need for rational sub grouping.
  • Non-normal data can sometimes also indicate the sub-groups exists.

Steps to develop Rational Subgroups

  • Identify the right data (Quality characteristics that are important to customers) to capture or track.
  • Select the source of data and the categories like shift, material, machine etc. 
  • Produce elements of the subgroup in closely similar identical ways and determine the range of variation within the subgroup.
  • Select the best sample data for subgrouping to get the desired control chart.
  • Use the ANOVA test to confirm the statistical difference between sub-groups.

Example of Rational Subgroup

Example: There are four bottle filling machines in XYZ Inc. Material quantity is one of the critical quality characteristics. Below are the different methods of subgrouping the data for the control chart.

Rational Sub Grouping

Case1: The first subgroup consists of 4 bottles from machine 1, the second subgroup consists of 4 bottles from machine 2, the third subgroup consists of 4 bottles from machine 3, and fourth subgroup consist of 4 bottles from machine 4.

Case2: Form a subgroup consists of one bottle each from four machines.

Case3: Select each subgroup to have a mix of four bottles from the machines.

Compute Range chart, standard deviation, and average chart for all the above three scenarios and determine the best subgrouping method.

Rational subgroups of individual control charts

Often rational subgrouping is a little bit unclear when dealing with individual control charts. In other words, when subgroup size is one. The basic idea underlying control charts of all types is to identify the capability of the process. The mechanism by which this is accomplished is the careful formation of rational subgroups.

Additional Rational Sub Grouping Resources

Subgrouping Video


Comments (3)


There is a question I’m not sure how to answer. Can you help?

Which of the following definitions are NOT associated with a rational subgroup?
A. Homogeneous
B. Essentially alike
C. Product produced approximately at the same time
D. One part is produced every hour

I was thinking that D is the answer, but wanted to hear thoughts.

I agree that D makes the most sense, Jennifer. We’re looking for items created within a “snapshot” of the process over a very small amount of time. And D would NOT fit that description.

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