Download Dr. Phil Rosenkrantz

American Society for Quality
The Importance of Understanding Type I and Type II Error in Statistical
Process Control Charts
Presented by: Dr. Phil Rosenkrantz
Statistical methods that involve sampling of any kind are subject to two kinds of sampling error: Type I
error is when your sample or test leads you to conclude a process is bad but it is really acceptable (a.k.a
“producer’s risk” or a “false alarm”). Type II error is when your sample or test leads you to conclude that
a process is operating OK but it is really not (a.k.a “consumer’s risk” or “failure to detect”). Wrong
conclusions lead to incorrect actions. In the world of quality the bigger concern is almost always Type II
error such as thinking a part, batch or process is good when it is really not. However, a major exception to
this thinking is found in the use of statistical process control (SPC) charts where Type I error (false
alarms) can cause people to hunt for problems that don’t exist which destroys user confidence and
damages the implementation of SPC very quickly. In my consulting experience this is a common problem
that is largely misunderstood. The major culprits are usually the decision rules being used to alert that the
process is out-of-control. The focus of this presentation is Type I error.
The goals of this presentation are to:
 Provide a brief review of the concepts of process control and process capability
 Explain Type I and Type II error with colorful examples
 Give examples of Type I and Type II error for common decision rules
 Illustrate how the improper use of decision rules creates excessive Type I error and creates
mistrust in the use of SPC
 Suggest simple approaches for reducing Type I error in SPC
Background
Dr. Phil Rosenkrantz is Professor Emeritus of Industrial & Manufacturing
Engineering at Cal Poly Pomona where he has been teaching and consulting for
34 years and served as Department Chair from 1990-1997. He was also founding
Coordinator of the MSQA Program at CSU Dominguez Hills from 1986-1989.
Phil was an engineering supervisor for General Motors prior to entering
academia. Educational background includes Doctor of Education in
Organizational Leadership from Pepperdine University; MS in Statistics from
UC Riverside; MS in Industrial Administration from Purdue University; and B.S.
in Mechanical Engineering from Kettering University (formerly GMI). P.E.
(California). 2005 Recipient of the Simon Collier Award. George P. Hart Award
for Outstanding Faculty Leadership 1999. Outstanding Teacher Award for the
College of Engineering 2012. Phil has been active in ASQ and ASEE (American Society for Engineering
Education). Over the years Phil has been a volunteer with the Boy Scouts of America (8 years), the Board
of Directors of Eastside Christian Schools (27 years), and U.S. Forest Service (20 years).