2. Chapter 8 – Rational Subgrouping
Eng. Mgt. 385
Statistical Process Control
Stephen A. Raper
Chapter 8 – Rational Subgrouping

3. Chapter 8 – Rational Subgrouping
Previous chapters only indicate a subgroup of some size, usually 3 –5 is necessary for drawing samples.
No basis was given for how to draw the samples.
Two overall approaches are suggested:
By order of subgroup
Other means
Consists of product all produced as nearly as possible at one time
Consists of product intended to be representative of all the production over a given period of time
Other Means
Convenience, cost, process determinate

4. Chapter 8 – Rational Subgrouping
Basic rule: pull samples in such a way as to maximize chances for detecting assignable cause of variation.
Basic Truth: know the system and process intimately.

5. Chapter 8 – Rational Subgrouping
Variation Exists:
Lot-to-Lot
Stream-to-stream
Time-to-time
Piece positional variation
Error of measurement
Equipment error
Human error
Understand variation exists, can impact process parameters!

6. Chapter 8 - Rational Subgrouping
Reproducibility
Accuracy
Precision
Gauge Studies

7. Program Completed University of Missouri-Rolla
Copyright 2001 Curators of University of Missouri