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Rational Subgrouping is a process of grouping items that were produced under the same set of conditions to measure the variation between the subgroups and within Subgroups. It is a “snapshot” of the process over a very small-time window i.e. samples taken in a time-ordered sequence. The variation measured within a Subgroup is Common Cause variation. The variation measured between Subgroups is Special cause variation. A Rational Subgroup helps an excellence practitioner to differentiate between common cause and special cause variations in a control chart and make corrective decisions when the charts are plotted using a sample of the data from the process. Without utilising the concept of Rational Subgroup, I feel a excellence practitioner would not be able to: Make correct decisions using control charts plotted with a sample of the process data. Correctly distinguish between common cause and special cause variations Example of Ration subgroup creation (1): Consider two Machines – X and Y with Operators 1 and 2 respectively producing a product say, metal disks. Rational Subgroup 1 will be, say, 10 random disks picked from Machine-X Operator 1 in the 1st of production. Rational Subgroup 2 will be, say, 10 random disks picked from Machine-Y Operator 2 in the 1st of production. (2): Consider two Machines – X and Y and Shifts A and B and the following production schedule: Machine X -> Shift A -> Operator 1 Machine X -> Shift B -> Operator 2 Machine Y -> Shift A -> Operator 3 Machine Y -> Shift B -> Operator 4 Rational Subgroup 1 will be, say, 10 random disks picked from Machine-X Shift A Operator 1 in the 1st of production. Rational Subgroup 2 will be, say, 10 random disks picked from Machine-X Shift B Operator 2 in the 1st of production. Rational Subgroup 3 will be, say, 10 random disks picked from Machine-Y Shift A Operator 3 in the 1st of production. Rational Subgroup 4 will be, say, 10 random disks picked from Machine-Y Shift B Operator 4 in the 1st of production.

Question: What would an excellence practitioner lose if he does not utilise the concept of rational subgrouping in the pursuit of process improvement? Answer: Rational subgrouping is set of observations made under the similar set of production environment. Sample or subgroup is in a way same only. Sample is subgroup of the population. But the only difference is when different samples are picked from the same population. Rational subgrouping reflects the process that you are working. Also, It usually represents the in-built variation of the process, which is called as common cause variation or within subgroup variation. It will tell you how the data been collected. The individual data points collected is usually independent of each other though it is all sub grouped. For Eg. If the coder produces 20 charts per hour and 50% of the chart to be audited. Hence the Auditor picks systematically every even numbered charts for auditing throughout the day or at even regular intervals. Sample of 10 per hour represents a subgrouping. Sample usually denotes a individual sample of the group. Sub group would always represent homogenous condition’s data points. Two types of variation: Within subgroup Variation: variation within the group / process sampled. Also called as common cause variation. Between subgroup variation: variation between the measurements of the sub groups. Also called as special cause variation, which has to eliminated for any quality improvement project. Control limits & Variations: Control limits are calculated using common cause variation. Sub group averages and variances are calculated from the homogenous set of data points. Subgroup is represented as individual point on the control chart with control limits inhibiting common cause variation. The goal of the project is eliminating the special cause variation and reducing the common cause variation. Usability: Sub grouping is done to help in decision making using the sample for the population. · Helps to reduce the common cause variation and eliminate the special cause / assignable cause variation. · Sub grouping along with control charts will create the base of decision making. · Helps us to find out how much of variation exists within subgroup. · Helps us to visible the process. When Rational subgrouping is not possible? When each and every items are checked or 100% audited, the rational subgrouping is of no use. For Eg. In case of automated inspection method / technology built audit method followed while processing and submitting a chart, rational subgrouping is of no use. Since all the charts are getting inspected. When medication effect is analyzed in preparation of medicine in pharmaceutical company or hospital, repeated analysis is done. Hence rational subgrouping is no use. In conclusion, in any business excellence project, if rational subgrouping is not done, common cause will be identified and reduced. This subgrouping will help identify the types of variation and the context of data like how they are collected, how time ordered the data is and highlights the variations and helps eliminating the special causes. Thanks Kavitha

Concept of rational subgrouping plays a very important role in pursuit of process improvement. Excellence practitioner will loose some important aspects if he ignore this concept. The method used to select samples for a control chart must be logical or rational. In the case of x bar and r chart, it is desirable that x bar chart detects a process shift, while the r chart should capture only common cause variation. That means there should be a high probability of variation between successive samples while the variation within the sample is kept low. There will be no exact idea of central tendency and dispersion of process over time. It means excellence practitioner will not be able first d out the answer of follow questions: 1. Has a special cause of variation caused the central tendency of this process to change over the time period observed or not? 2. Has a special cause of variation caused the process distribution to become more or less consistent? Rational subgrouping is the base of control charts. Rational subgroups are composed of items which were produced under essentially the same conditions. The subgroup provides a snapshot of the process at that moment in time. A fundamental aspect of the subgroup is thus to estimate the common cause variation within the process, since the within subgroup variation is used to define the width of the control limits. Therefore it is critical that the causes of within subgroup variation be representative of the causes of variation between subgroup. Is absence of rational subgrouping practices will not able to find out short term within subgroup variation which leads to non prediction of longer term between subgroup variation, causing g the statistically uncontrolled. When the longer term variations is not predicted by shorter term within subgroup variation, then a special cause has been identified but in this case of absence of rational subgrouping central tendency is not defined exactly so special cause will not be the correct to resolve the problem.