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.

1. 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 Wang
Presented live on Mar 09th, 2011
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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

6.  Some well-defined, commonly-used reliability
indices
• Reliability
• Failure Rate
• MTTF / MTBF
• …
Convention Reliability Metrics

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