When you adopt a non convention reliability metric
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Transcript of When you adopt a non convention reliability metric
When You Adopt a Non‐convention Reliability
Metric …(当你采用自定义的可靠性度
量 …)
Dr. Wendai Wang (汪文岱)©2011 ASQ & Presentation WangPresented live on Mar 09th, 2011
http://reliabilitycalendar.org/The_Reliability_Calendar/Webinars_‐_Chinese/Webinars_‐_Chinese.html
http://reliabilitycalendar.org/The_Reliability_Calendar/Webinars_‐_Chinese/Webinars_‐_Chinese.html
ASQ Reliability Division Chinese Webinar
SeriesOne 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 events
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
Key Words/Terminologies 关键词/术语
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
Design in and Report out
Reliability Metrics – theoretically & practically
• define / represent;
• be able to be designed in;
• report out
the reliability of your systems, subsystems and parts.
Convention Reliability Metrics
Some well-defined, commonly-used reliability
indices
• Reliability
• Failure Rate
• MTTF / MTBF
• …
Annual Failure Rate ?
Annualized Failure Rate (AFR) to measure the parts
reliability – What ?
Failure Rate ?
Have been used in high-tech industries.
Could be just a Failure Rate? But confused with the
word “annualized”.
Research
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
Non-convention Metric
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.
Definition
The AFR definition we used:
AFR = Total Failures from Units in the Denominator
Total Units Shipped in past Warranty Period
Good for Business
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 !
Disadvantages
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
Case Study
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.
Misunderstand in Calculation
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 Terminology
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.
The Language of Engineering is Math.
What’s the term we really defined?
Theoretically, it’s an average value of the
unreliability function over the warranty period *.
N
tF
N
n
AFR
N
i
i
1
Period Warranty Past in Shpipped Units Total
rDenominato the in Units from Fai lures Total
T
dttFT
AFR0
)(1
* Depends on the real calculation method.
Different Data Intervals
Precisely, it’s an estimator of an average value of the
unreliability function over the period.
Years
Probability of Failure
1 2 30 4
AFR Value with 2-year data
Part’s unreliability curve
AFR Value with 1-year data
Simple Explanation
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)
Build up Theoretical Base
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
Converted to Other Quantities
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
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%
Summary
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.