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

wendaiw@gmail.com

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.