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While continuous data is generally preferred over discrete data, please indicate circumstances where discrete is the preferred data type although continuous data is available for the same characteristic. Note for website visitors - Two questions are asked every week on this platform. One on Tuesday and the other on Friday. All questions so far can be seen here - https://www.benchmarksixsigma.com/forum/lean-six-sigma-business-excellence-questions/ Please visit the forum home page at https://www.benchmarksixsigma.com/forum/ to respond to the latest question open till the next Tuesday/ Friday evening 5 PM as per Indian Standard Time. The best answer is always shown at the top among responses and the author finds honorable mention in our Business Excellence dictionary at https://www.benchmarksixsigma.com/forum/business-excellence-dictionary-glossary/ along with the related term.

Rational Subgrouping It is the rational or logical way in which data are organised into subgroups to be used in process charts. Subgroups can be used to classify two types of variation in a process –within subgroup and the between the subgroup. 1. Within Subgroup - variation seen within a subgroup ; This is also called common cause variation 2. Between Subgroup – variation observed between subgroups due to one-off factors/conditions or special factors/conditions ; otherwise known as special cause variation Upper and Lower control limits in a control chart are calculated using the ‘within subgroup’ variation and hence it becomes imperative to choose a subgroup so that the process has a common cause variation. So here, the key objective in choosing a rational subgroup is to have minimal variation within subgroup. This is where an excellence practitioner would miss out, if he/she does not make use of the rational subgrouping method properly. Let us see with an example. In a banking IT support project, the support for the project is provided in 3 shifts. The team is a mixture of freshers and experienced professionals. Now there were so many defects that cropped out. The team did not have a clue as how to address these defects. Two rational subgrouping could be done – one on shift and the other with experience level. We need to find which one has minimal variation in ‘within subgroup’. Apart from that, we also need to eliminate ‘between the subgroups’ variations. Let us see with ‘Shift’ as a rational subgroup. There are 3 shifts. Shift1 is having an average 10 defects /day, Shift 2 has 10-12 defects/day and Shift 3 has on rare occasions, 20 defects/day and on other occasions 9-10 defects. Defects include cosmetic, high, severity 2 and severity 1 defects. While the shifts, Shift 1 and Shift2 do not have much difference, Shift 3 has a considerable variation at times. This is due to the fact that on rare occasions SME availability is not available for a key application. Now with the ‘Shift’ as a rational subgroup, shift 1 and shift 2 has minimal variation. To eliminate the special cause reason for SME unavailability, remove the dependency on the SME, by ensuring more SMEs are available in that shift. Spread out the SMEs across shifts. Let us see with ‘Experience Level’ as a rational subgroup. 3 categories. 1-2 years ; 2-5 years; 6-10 years. Team members with experience level – 1-2 years produced 14 defects on an average/day sometimes and 20 defects/day on an average on some other days, team members with experience level – 2-5 years produced 6 defects/day on an average sometimes and 15 defects/day on some other days, and team members with experience level 6-10 years produced 2-3 defects on an average and occasionally produced 5 defects . As we see the variation within subgroup is high and also variation between the subgroups – 1-2 years and 2-5 years is large. Eliminating the variation between the subgroups is not straight-forward. So the best subgroup in this case is ‘Shift’ which will have minimal ‘within the subgroup’ variation Conclusion Thus an excellence practitioner has to choose the right subgroup to ensure minimal within subgroup variation.