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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3. When You Adopt a Non-convention
Reliability Metric …
当你采用自定义的可靠性度量 …
汪文岱博士
可靠性工程总监
GreenVolts, Inc.
Fremont, California
wendaiw@gmail.com
4. Case Study: 案例研究
Failure Data Analysis: 失效数据分析
Field Reliability: 应用可靠性
Lesson Learned: 经验教训
Reliability Measures: 可靠性度量
Warranty Period: 保修期
Key Words/Terminologies
关键词/术语
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
7. Annualized Failure Rate (AFR) to measure the parts
reliability – What ?
Annual Failure Rate ?
8. Have been used in high-tech industries.
Could be just a Failure Rate? But confused with the
word “annualized”.
Failure Rate ?
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 Lycette
Research
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-convention Metric
11. The AFR definition we used:
Total Failures from Units in the Denominator
AFR =
Total Units Shipped in past Warranty Period
Definition
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. 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
How difference between methods?
Lots of misunderstanding
• Failure rate
Disadvantages
14. Always see reliability (in term of AFR) improvement
for design changes
• Not always match with reality
• Not see reliability improvement in whole parts pool
Discrepancy in an AFR value between our estimate
supplier’s estimate.
Case Study
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 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. The AFR we defined is not the Failure Rate !
BUT suppliers thought: our AFR = the Failure Rate.
AFR numbers arrived at widely different figures
even using the same data.
Misunderstand in Terminology
17. What’s the term we really defined?
Tota lFa i l ures
fromUni tsi nthe Denomi na to
r
AFR
Tota lUni tsShpi ppednPa s tWa rra ntyPeri od
i
N
n F t i
i 1
N N
Theoretically, it’s an average value of the
unreliability function over the warranty period *.
1 T
AFR F (t )dt
T 0
The Language of Engineering is Math.
* Depends on the real calculation method.
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 4
Different Data Intervals
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 data)
Simple Explanation
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 bounds
Build up Theoretical Base
21. Be able to establish relationship between AFR and
other reliability quantities.
Example: Part - PN 123456
• 489 units shipped in past 2 years.
• There were 56 failures among them.
• AFR_MME = 56 / 489 = 11.5%, which can be converted to
• FR = 13.47 FPMH
Converted to Other Quantities
22. Different estimate methods
• AFR_MME = 11.5% FR = 13.47 FPMH
• AFR_MLE = 14.1% FR = 17.69 FPMH
Failure Rate directly from data
• FR = 17.88 FPMH
Confidence bounds for AFR (at 90% CL)
• AFR_LL = 10.9%
• AFR_UL = 17.8%
Converted to Other Quantities
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