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Know Your Statistical Distribution: What to Do When It's Not a Bell Curve (MFG261C)

Presented by: William A. Levinson, P.E.
(*) Single User Price. For multiple users please call 1-844-384-4744
Pre Recorded Webinar
60 minutes
Event Description
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Create effective SPC charts and calculate accurate process performance indices for non-normal processes

Traditional statistical process control (SPC) and calculation of process performance indices rely on the textbook assumption that the critical to quality (CTQ) characteristic follows a normal distribution, or bell curve. If the processes on the factory floor do not cooperate with this assumption, control charts will not work properly while estimates of defects per million opportunities (DPMO) can be off by five or more orders of magnitude. That's when our customer might tell us, "Your 'Six Sigma' process is not even capable."

Textbook statistical process control (SPC) relies on the assumption that the process follows a bell curve or normal distribution. When the process does not cooperate with this assumption, the risk of false alarms (and consequent overadjustment) can be several times more than expected, and process performance indices can be grossly misleading.

This underscores the need to (1) test the assumption of a normal distribution and, if the distribution is not a bell curve, (2) identify the correct distribution and then calculate appropriate control limits and process performance indices. While we can never prove that a process follows any particular distribution, we can prove beyond a reasonable doubt (the Type I or alpha risk) that it does not follow a selected distribution.

Join this session by our expert speaker William A. Levinson to get a better understanding of statistical distribution and what to do when it’s not a bell curve. Learn about some qualitative and quantitative tests for normality (and other distributional assumptions).

Session Highlights:

  • Know that, contrary to the content of most textbooks and published articles, not all manufacturing processes follow the normal distribution.
  • Get awareness of qualitative (histogram, normal probability plot) and quantitative (Anderson-Darling test, chi square goodness of fit test) tests for normality, or goodness of fit to any other distribution.
  • Know when the distribution might be non-normal, e.g. when there is a one-sided specification limit at one end and a physical limit (such as the fact that we can't get fewer than zero impurities) at the other.
  • Know how control charts can be modified to behave exactly like familiar Shewhart control charts when the distribution is non-normal.
  • Know how to calculate process performance indices that reflect accurately the nonconforming fraction for the process.

Who should attend?
Quality engineers and technicians

About Our Speaker(s)

William A. Levinson | Quality and Productivity Management SpeakerWilliam A. Levinson P.E.
William A. Levinson, P.E., is the principal of Levinson Productivity Systems, P.C. He is an ASQ Fellow, Certified Quality Engineer, Quality Auditor, Quality Manager, Reliability Engineer, and Six Sigma Black Belt. He is also the author of several books on quality, productivity, and management, of which the most recent ... More info

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    Event Title: Know Your Statistical Distribution: What to Do When It's Not a Bell Curve
    Presenter(s): William A. Levinson, P.E.

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