Quality Progress - December 2016 - 61


STATISTICS ROUNDTABLE

BY NECIP DOGANAKSOY, GERALD J. 	
									HAHN AND WILLIAM Q. MEEKER

Identify and Act
Performing product life data analysis with unidentified subpopulations
THE NATIONAL Highway Traffic Safety

In a previous Statistics Roundtable col-

quality and reliability issues, involving

Administration (NHTSA) in 2015 recalled

umn, we showed how to identify subpopu-

large quantities of product at the end of

28 million vehicles equipped with Takata

lations with different failure vulnerabilities

the process that might be an appreciable

air bags due to inflators rupturing during

by using segmentation analysis-a divide-

time later.

deployment.1 It was determined that mois-

and-conquer strategy that breaks down the

ture could penetrate the inflator canister

total product population into meaningful

staff discovered a faulty valve in an early

and make the propellant more explosive

subpopulations that allows separate analy-

production stage diffusion furnace. Inves-

over time.

ses on each identified subpopulation. See

tigations showed that the valve malfunc-

"Segmentation Analysis of Bleed Systems,"

tioned sporadically over time, potentially

p. 64, for a summary of the column.

compromising dielectric insulation and

Data analysis showed that older air
bags and those in regions with high humid-

2

In a particular operation, manufacturing

leading to premature dielectric breakdown

ity, such as the Gulf Coast, were up to 10
times more likely to rupture. This informa-

Subpopulation identification

(that is, sudden loss of electrical insula-

tion about product subpopulations with

In segmentation analysis, the definition

tion) and device failure of units processed

different air bag malfunction vulnerability

of subpopulations should be based on

during the affected periods of time.

played a key role in defining short term

physical considerations. This requires

and long-term remedial strategies.

an in-depth understanding of the design,

tion was addressed vigorously and steps

manufacture and use conditions of the

were taken to avoid its recurrence. In

situations in which some units are more

product-and an identification of the spe-

the meantime, however, an appreciable

likely to fail in service than others. Such

cific subpopulation to which a particular

quantity of product had been built, some

differences in reliability may be due to vari-

product unit belongs. In the sidebar ex-

of which was shipped to customers. The

ability in manufacturing-process conditions,

ample, the base at which each aircraft was

rest remained in inventory. It is these

or in raw materials and components. To take

stationed was used to segment the data.

untouched, potentially vulnerable devices

Similar to this example, there are many

appropriate action to minimize premature

In some applications, however, relevant

After it was discovered, the malfunc-

that were the target of this study.
Preliminary evaluations suggested

failures in such situations, it is important to

product subpopulations cannot be identi-

know which parts of the product population,

fied on an individual unit basis. As a result,

that-due to the sporadic nature of the

already built, are most vulnerable.

you cannot conduct a segmentation analy-

malfunction-only an unknown fraction

sis. In this column, we describe a method

(p) of a defined population of product had

to analyze life data in such situations.

been exposed to the faulty valve and was

In addition, identifying vulnerable
product subpopulations can help identify

vulnerable. It was not possible, however,

the root causes of reliability problems
and allow you to use this knowledge to

Semiconductor example

to identify individual affected devices. Yet

improve future product reliability. Targeted

Description. A manufacturing process for

for various reasons, as indicated in the

experimentation, coupled with appropriate

semiconductor devices requires hundreds

following points, an estimate of the value

analysis of resulting and existing data, can

of steps, often spanning weeks or even

of p was needed:

help you identify product subpopulations

months. Missed problems in early stages

*	 First, a decision was required about

with different levels of vulnerability.

of manufacturing can lead to significant

the disposition of product already
shipped to customers. In particular, if p

NEW STATISTICS BOOK A new ASQ Quality Press book features a

collection of 90 Statistics Roundtable columns previously published in QP.
Statistical Roundtables, edited by Christine Anderson-Cook and Lu Lu, is broken into nine chapters on topics such as statistical engineering, data quality
and measurement, data collection and key statistical tools. The 480-page book
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exceeded 3%, a 100% recall was deemed
necessary.
*	 A second reason pertained mainly to
the units still in the producer's inventory, but also applied to units returned
from the field. A burn-in life test at

December 2016 * QP 61


http://www.tinyurl.com/stats-roundtables

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