A review of how we sub-divide data for analysis using rational sub-grouping.

1. Section & Lesson #:
Pre-Requisite Lessons:
Complex Tools + Clear Teaching = Powerful Results
Rational Sub-Grouping
Six Sigma-Measure – Lesson 18
A review of how we sub-divide data for analysis using rational sub-
grouping.
Six Sigma-Measure #17 – Variation Over Time (Short/Long Term
Data)
Copyright © 2011-2019 by Matthew J. Hansen. All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted by any means
(electronic, mechanical, photographic, photocopying, recording or otherwise) without prior permission in writing by the author and/or publisher.

2. o What is rational sub-grouping?
• It’s the logical distinction of potential sub-processes that exist within an overall process.
• These sub-processes are usually distinguished (or grouped) by a factor or category. Examples:
 Time (e.g., shift, time of day, day of month, etc.)
 Location (e.g., country, geographic region, metro/rural, native language, etc.)
 Processes (e.g., separating different processes or different methods of the same process, etc.)
 People (e.g., tenure, education level, etc.)
o How do I know if I need rational sub-grouping?
• Special cause variation may reveal the need
for rational sub-grouping.
 If the same special cause tests tend to fail, then
a sub-group may exist in the process.
• Non-normal data can sometimes also indicate
that sub-groups exist.
 A non-normal process may be caused by multiple,
overlapping normal processes.
 From a normality plot, group the data points for
each straight line in the plot to see if sub-groups exist.
o How do I know if I have the right rational sub-grouping?
• Use ANOVA and HOV tests (Analyze phase) to confirm statistical differences between sub-groups.
 Keep the sub-grouping only if a statistical difference exists (low P value) between the sub-groups.
Rational Sub-grouping Defined
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125.0%100.0%75.0%50.0%25.0%0.0%-25.0%-50.0%
99.9
99
95
90
80
70
60
50
40
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20
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5
1
0.1
MetricB
Percent
M
S
N
A
P
Probability Plot of MetricB
Normal
Sub-Group A
Sub-Group B

3. Practical Application
o Identify at least 3 metrics used by your organization that are continuous values.
• For each metric, identify at least 3 types of sub-groups currently used for reporting.
• What are at least 2 other types of sub-groups that are possible, but improbable?
 How can you confirm if it’s a rational sub-group that should be separately measured?
Copyright © 2011-2019 by Matthew J. Hansen. All Rights Reserved. No part of this publication may be
reproduced, stored in a retrieval system, or transmitted by any means (electronic, mechanical, photographic,
photocopying, recording or otherwise) without prior permission in writing by the author and/or publisher.
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