/k

Introduction to SPC

2WS10 Applied Statistics

A. Di Bucchianico

/k

What is Statistical Process Control (SPC )?

The set of operational and m

anagerial activities that a company uses

to ensure that the quality characteristics of a product are at the

required level??

A series of control charts??

/k

Statisticalm

ethods

•acceptance sampling (AS)

•statistical process control (S

PC)

•design of experiments (DOE)

/k

Dimensions of Quality

Perform

ance

Reliability

Durability

Serviceability

Aesthetics

Features

Perceived Q

uality

Conform

ance to standards

/k

Definitions of Quality

Fitness for use

–design

–conform

ance

Inversely proportional to variability

/k

Why study history of quality control?

•understand and foresee trends and future

developments

•companies m

ay still be w

orking w

ith concepts from

the past

/kHistory of Quality control

•craftsmen: quality is w

hat producer and consumer agree

about

•start of mass-production

–parts should be exchangeable

–all the same

–no variability

/kHistory: in the beginning

Quality really became an issue during the Industrial

revolution (19th century).

Inspection-oriented quality w

as adopted in the emerging

factories (usually through inspection department)

F.W

. Taylor startedresearch on quality and m

anagement in

the 1890’s (for more inform

ation, read the introductionto the

Taylor collection at the Stevens Institute of Technology)

/k

output

input

processes

specifications scrap

customer

History part I: inspection

•inspection just before shipping of product

•related definition of quality:

–conform

ance to specifications

/kProblems with total inspection

•mistakes have already been m

ade

–“one cannot inspect quality into a product”

•does not yield insight in production process

•very expensive

•inspection is not water-tight

/k

How many times does the letter foccur ?

Finished files are the result of years of scientific

study combined w

ith the experience of many

years.

/k

How many times does the letter foccur ?

Finished files are the result ofyears of

scientific study combined w

ith the

experience ofmany years.

/k

Example of acceptance sampling

sim

plest form

:

–lot size N

–sample size n

–acceptance number c (lot is accepted if number of defectives in sample

is ≤c)

OC (Operating characteristic)-curve gives

P(acceptance of lot | percentage defectives)

/k

input

processes

output

scrap

customer

specifications

sampling plans

History part II: Acceptance sampling

/k

History: W.A. Shewhart

Shewhart developed at Bell Labs in the 1920’s a process

oriented control approach.

Instead of inspection, production process is m

onitored.

Sampling plans w

ere used to achieve acceptable quality levels

(AQL).

Techniques employed in quality control were statistical,

basically sampling m

ethods and control charts

/k

History part III: Monitoring process output

output

input

OCAP

control

charts

limits

single process

/kSPC: Statistical process control

•new important tool: control chart

•measurements of production process during production

•prevention instead of detection afterw

ards

•monitoring variance behaviour of production

•corresponding definition of quality:

–variation of process fits within tolerances

X-b

ar C

har

t fo

r yie

ld

Subgro

up

X-bar

04

812

16

20

13,6

13,814

14,2

14,4

/k

History: World War II

Increase of production during W

orld W

ar II caused a large

expansion of quality control activities in both the U.S.A. and

Britain.

Statistical techniques w

ere so effective that they w

ere

classified as m

ilitary secrets during W

orld W

ar II!

After World W

ar II, the U.S.A. only had m

ajor production

facility left.

Lackof competition andenorm

ous post-war demand caused

disinterest in quality control.

/k

History: Japan

Before W

orld W

ar II, Japanese products w

ere cheap but of

low quality.

After World W

ar II, Japan had to rebuild its factories

(including the m

anagement structure).

American consultants (in particular Deming and Juran) were

brought in to teach quality.

A national centre for quality control was established (JU

SE)

and m

any national activities w

ere organized.

Japanese companies have extensive training programme for

“everyone”in the company.

/k

History: W.E. Deming

Deming applied Shewhart’s

principles of quality control at

various places

From 1950 on, he often visited Japan as lecturer and

consultant (the Japanese honoured him

by naming the highest

Japanese quality award after him

)

In spite of this, his fame in the US did not really start until

1980.

Deming puts m

uch emphasis on m

anagement rather than

production floor.

/k

History: J.M. Juran

Early Japanese efforts in quality control overemphasised

statistical methods under the influence of Deming.

Juran’s 1954 visit to Japan initiated a change of focus on

technology to a focus on m

anagement (especially

involvement of top-m

anagement).

Juran also emphasised the importance of education and the

cost of quality.

/k

History part IV: Problem solving tools

7 problem solving tools (‘M

agnificent Seven’)

–fishbone/Ishikawa diagram

–Pareto chart

–run chart

–defect concentrationdiagram

–Box-andWhiskerplot

–...

/k

History: K. Ishikawa

Ishikawa w

as very important to the quality revolution in

Japan because of his m

any activities (consulting, member of

various committees, ...).

Ishikawa stressed that quality control must try to m

eet the

requirements of customers.

Technical contribution: the cause-and-effect (or fishbone)

diagram.

/k

Process thinking

•all deviations from product specifications have causes in production

process

•outcome production process is caused by m

any process factors

Methods/

adjustments

People

Environment

Tools

Mach

ine

Materials

deviation

/k

Design of Experiments

•preferably

designphase(cf. EVOP)

•off-line

•team

•detect causes of variation

•optimise process

/k

History: G.E.P. Box

•IntroducedDOE in industrialenvironment sinceearly1950’s

•Developedresponse surfacemethodsat Im

perial Chemical

Industries

•Movedto US in 1960 and initiatedCenterfor Quality and

Productivity Im

provementin W

isconsin

•ASQC history: http://w

ww.asq.org/join/about/history/box.htm

l

•George Box award:

http://w

ww.enbis.org/barcelonaconference/gepbox.htm

l

/k

History: Taguchi

Systematic experimentation is necessary for product

development that leads to high quality products

Taguchi developed a system of statistical experimentation

that meet the needs of the engineer working in industry and

is easier to understand than the classical statistical methods

Taguchi introduced the concept of “loss function”(i.e., how “

much ”

do I conform

to specifications?) and “robust design”.

/kConform

ance to specifications

Why is product just outside specifications w

orse than product just

inside specifications?

classical: yes/no

LS

LU

SL

LS

LU

SL

modern: Taguchi loss function

/kConform

ance

to specificationsII

specificationsof characteristic1

specificationsof characteristic2

/k

History: US rediscovers quality control

In the 1970’s and 1980’s Japan established itself as w

orld leader

in quality and became an economic superpower

Several American industries could not compete w

ith Japanese

industries, because of their neglect of quality control

Quality control starts to prosper again in the 1980’s in the US

under the guidance of Deming, Juran and new names like

Feigenbaum

and Crosby.

Examples of successful American companies include Ford

( QS9000) and M

otorola (Six Sigma).

ASQ: American Society forQuality

/k

History: Europe

Interest in quality control in Europe did not start until the

1990’s.

Much emphasis in Europe on ISO 9000certification.

European quality control initiatives like EFQM

focus m

ainly

on m

anagement issues.

In 2000, ENBIS

(EuropeanNetw

ork

forBusiness and

Industrial Statistics) was founded. InitiativebyBisgaard,

thensuccessorof Box in W

isconsin.

/k

History part V: Monitoring process input

•Control of process factors

•Prevention of disturbances

•Tools: SPC, TPM, Poka Yoke, ...

•corresponding definition of quality:

–variation of process fits w

ithin tolerances

PROCESS

output

specification

s

input

/k

History: new developments

An important new development is the Six Sigma approach

developed by M

otorola (one of the first winner of the Baldrige

Award)

Six Sigma approach uses known statistical and m

anagement

concepts to come to an overall approach to quality:

–use statistical tools (DOE, regression analysis) to understand

relations betw

een input and output parameters of processes

–use control charts on inputs

instead of outputs

–direct link of effortswithreductionof costs

/k

His

tory

part

VI: Q

ualit

y b

y d

esig

n

Actions in design phase:

–prevent quality problems in production phase

–prevent quality problems w

hen customer uses product

Typical tools: Design of Experiments, Robust Design

Corresponding definition of quality:

–quality is conform

ance to customer requirements and

expectations

/k

The m

ain line of history to be remembered is

•total inspection

•acceptance sampling

•control charts on output

•control charts on input

•technical solutions/ TPM / ...

•quality by design

Sum

mary

of his

torica

l deve

lopm

ent

insp

ecti

on

pre

ven

tion