When you Adopt a                       Non‐convention                      Reliability Metric                             ...
ASQ Reliability Division                   English Webinar Series                   One of the monthly webinars           ...
When You Adopt a Non-convention                 Non-                 Reliability Metric …          Wendai Wang, Ph.D.     ...
GreenVolts’GreenVolts’ CPV System
Reliability Metrics – theoretically & practically   • define / represent;   • be able to be designed in;   • report out th...
Some well-defined, commonly-used reliability indices  • Reliability  • Failure Rate  • MTTF / MTBF  • …Convention Reliabil...
Annualized Failure Rate (AFR) to measure the parts reliability – What ?Annual Failure Rate ?
Have been used in high-tech industries. Could be just a Failure Rate? But confused with the word “annualized”.Failure Rate ?
Some studies / publications  • “AFR: Problems of Definition, Calculation and    Measurement in a Commercial Environment” b...
It’s unconventional  • Different definitions (self defined)  • No standard calculation method  • Likely result in signific...
The AFR definition we used:            Total Failures from Units in the Denominator    AFR =            Total Units Shippe...
Apparently it’s simple  • Easy to obtain (from data)  • Seems easy to understand     o Failure percentage based Seems mean...
Lack of theoretical base  • What does it really mean?  • How to convert the AFR to other reliability quantities?  • How to...
Always see reliability improvement (in term of AFR)  for a new design (part) or a design changes  •   Not always match wit...
The AFR value highly depends on the interval over which the data is collected.  • Warranty period is in the definition.  •...
The AFR we defined is not a Failure Rate ! Suppliers thought: our AFR = the Failure Rate. AFR numbers arrived at widely di...
What’s the term we really defined?                Total Failures from Units in the Denominato r        AFR =              ...
Precisely, it’s an estimator of an average value of the unreliability function over the period.                 Probabilit...
Numerical example*  • Failure Rate (constant) = 0.1 failures/year  • New order = 1,000 parts/year  • AFR = 5.25% (using Ye...
A percentage based metric A non-parametric estimate  • matching moment estimation (MME) A better estimate (MLE) could be i...
Establish relationship between AFR and other reliability quantities. Example: Part – P/N 123456  • 489 units shipped in pa...
Different estimate methods  • AFR_MME = 11.5% ↔ λ = 13.47 FPMH  • AFR_MLE = 14.1% ↔ λ = 17.69 FPMH Failure rate directly f...
Non-conventional reliability metrics were wisely defined for good reasons. A thorough study is often needed to make good s...
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When you adopt a non convention reliability metric

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Most commercial enterprises want some meaningful but simple metrics to measure, track and report their part and/or product reliability, which link well to their business and/or engineering processes. An internal reliability measurement may sometimes be "defined" for that purposes. However, without careful consideration initially, the adopted non-conventional reliability metric may be lack of scientific sense, causing a lot of confusion and misunderstanding in calculation, reporting and communication. This talk presents you a real case study, which turns out to be a good example of lesson learned.

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When you adopt a non convention reliability metric

  1. 1. When you Adopt a  Non‐convention  Reliability Metric Wendai Wang ©2011 ASQ & Presentation Wendai Wang Presented live on Jul 14th, 2011http://reliabilitycalendar.org/The_Reliability_Calendar/Webinars_‐_English/Webinars_‐_English.html
  2. 2. ASQ Reliability Division  English Webinar Series One of the monthly webinars  on topics of interest to  reliability engineers. To view recorded webinar (available to ASQ Reliability  Division members only) visit asq.org/reliability To sign up for the free and available to anyone live  webinars visit reliabilitycalendar.org and select English  Webinars to find links to register for upcoming eventshttp://reliabilitycalendar.org/The_Reliability_Calendar/Webinars_‐_English/Webinars_‐_English.html
  3. 3. When You Adopt a Non-convention Non- Reliability Metric … Wendai Wang, Ph.D. Reliability Engineering GreenVolts, Inc. Fremont, California
  4. 4. GreenVolts’GreenVolts’ CPV System
  5. 5. Reliability Metrics – theoretically & practically • define / represent; • be able to be designed in; • report out the reliability of your systems, subsystems and parts.Design in and Report out
  6. 6. Some well-defined, commonly-used reliability indices • Reliability • Failure Rate • MTTF / MTBF • …Convention Reliability Metrics
  7. 7. Annualized Failure Rate (AFR) to measure the parts reliability – What ?Annual Failure Rate ?
  8. 8. Have been used in high-tech industries. Could be just a Failure Rate? But confused with the word “annualized”.Failure Rate ?
  9. 9. Some studies / publications • “AFR: Problems of Definition, Calculation and Measurement in a Commercial Environment” by Jon Elerath • “The iFR Method for Early Prediction of Annualized Failure Rate in Fielded Products” by Bill LycetteResearch
  10. 10. It’s unconventional • Different definitions (self defined) • No standard calculation method • Likely result in significantly different estimates by customers and supplies It could be • a failure percentage based or • a time based rate.Non-Non-convention Metric
  11. 11. The AFR definition we used: Total Failures from Units in the Denominator AFR = Total Units Shipped in past Warranty PeriodDefinition
  12. 12. Apparently it’s simple • Easy to obtain (from data) • Seems easy to understand o Failure percentage based Seems meaningful for business process Seems well defined (in calculation) Not a failure rate !Good for Business
  13. 13. Lack of theoretical base • What does it really mean? • How to convert the AFR to other reliability quantities? • How to quantify the confidence bounds? Confusion in calculations Lots of misunderstanding • Is it the failure rate?Disadvantages
  14. 14. Always see reliability improvement (in term of AFR) for a new design (part) or a design changes • Not always match with reality • Not see reliability improvement in whole parts pool Discrepancy in an AFR value between our estimate and supplier’s estimate.Case Study
  15. 15. The AFR value highly depends on the interval over which the data is collected. • Warranty period is in the definition. • It may take as much as a whole period before the improvement (or degradation) is observed. Someone couldn’t wait and reported out AFR values based on a short period (available data) !Misunderstand in Calculation
  16. 16. The AFR we defined is not a Failure Rate ! Suppliers thought: our AFR = the Failure Rate. AFR numbers arrived at widely different figures even using the same data.Misunderstand in Terminology
  17. 17. What’s the term we really defined? Total Failures from Units in the Denominato r AFR = Total Units Shpipped in Past Warranty Period N n ∑ F (t ) i =1 i = = N N Theoretically, it’s an average value of the unreliability function over the warranty period *. 1 T AFR = ∫ F (t ) dt T 0The Language of Engineering is Math. * Depends on the real calculation method.
  18. 18. Precisely, it’s an estimator of an average value of the unreliability function over the period. Probability of Failure AFR Value with 2- Part’s year data unreliability curve AFR Value with 1- Years year data 0 1 2 3 4Different Data Intervals
  19. 19. Numerical example* • Failure Rate (constant) = 0.1 failures/year • New order = 1,000 parts/year • AFR = 5.25% (using Year 2010 data only) • AFR = 9.56% (using 2-year, 2009 and 2010, data)Simple Explanation
  20. 20. A percentage based metric A non-parametric estimate • matching moment estimation (MME) A better estimate (MLE) could be introduced • fully use information from the data • Independent of data interval • be able to establish the confidence boundsBuild up Theoretical Base
  21. 21. Establish relationship between AFR and other reliability quantities. Example: Part – P/N 123456 • 489 units shipped in past 2 years. • There were 56 failures among them. • AFR = 56 / 489 = 11.5%, which can be converted to • Failure Rate: λ = 13.47 FPMH.Converted to Other Quantities
  22. 22. Different estimate methods • AFR_MME = 11.5% ↔ λ = 13.47 FPMH • AFR_MLE = 14.1% ↔ λ = 17.69 FPMH Failure rate directly from data • λ = 17.88 FPMH Confidence bounds for AFR_MLE (at 90% CL) • AFR_LL = 10.9% • AFR_UL = 17.8%Converted to Other Quantities
  23. 23. Non-conventional reliability metrics were wisely defined for good reasons. A thorough study is often needed to make good sense out of it. Communication is imperative. Education still is a crucial task for reliability engineering.Summary

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