Introduction to SPC

2WS02 Industrial Statistics

A. Di Bucchianico

What is Statistical Process Control (SPC )?

The set of operational and managerial activities that a company uses

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

required level??

A series of control charts??

Statistical methods

•acceptance sampling (AS)

•statistical process control (SPC)

•design of experiments (DOE)

Dimensions of Quality

Performance

Reliability

Durability

Serviceability

Aesthetics

Features

Perceived Quality

Conformance to standards

Definitions of Quality

Fitness for use

– design

– conformance

Inversely proportional to variability

Why study history of quality control?

• understand and foresee trends and future

developments

• companies may still be working with concepts from

the past

History of Quality control

•craftsmen: quality is what producer and consumer agree

about

•start of mass-production

– parts should be exchangeable

– all the same

– no variability

History: in the beginning

Quality really became an issue during the Industrial revolution

(19th century).

Inspection-oriented quality was adopted in the emerging

factories (usually through inspection department)

F.W. Taylor started research on quality and management in

the 1890’s (for more information, read the introduction to the

Taylor collection at the Stevens Institute of Technology)

outputinput

processes

specifications

scrap

customer

History part I: inspection

inspection just before shipping of product

related definition of quality:

conformance to specifications

Problems with total inspection

• mistakes have already been made

– “one cannot inspect quality into a product”

• does not yield insight in production process

• very expensive

• inspection is not water-tight

Example of acceptance sampling

simplest 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)

input

processes

output

scrap

customer

specifications sampling plans

History part II: Acceptance sampling

History: W.A. Shewhart

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

oriented control approach.

Instead of inspection, production process is monitored

Sampling plans were used to achieve acceptable quality

levels (AQL)

Techniques employed in quality control were statistical,

basically sampling methods and control charts

History part III: Monitoring process output

outputinput

OCAP control charts

limits

single process

SPC: Statistical process control

•new important tool: control chart

•measurements of production process during production

•prevention instead of detection afterwards

•monitoring variance behaviour of production

•corresponding definition of quality:

– variation of process fits within tolerances

X-bar Chart for yield

Subgroup

X-b

ar

0 4 8 12 16 2013,6

13,8

14

14,2

14,4

History: World War II

Increase of production during World War II caused a large

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

Britain.

Statistical techniques were so effective that they were

classified as military secrets during World War II!

After World War II, the U.S.A. only had major production facility

left.

Lack of competition and enormous post-war demand caused

disinterest in quality control.

History: Japan

Before World War II, Japanese products were cheap but of low

quality.

After World War II, Japan had to rebuild its factories (including

the management structure).

American consultants (in particular Deming and Juran) were

brought in to teach quality.

A national centre for quality control was established (JUSE) and

many national activities were organized.

Japanese companies have extensive training programme for

“everyone” in the company.

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 much emphasis on management rather than

production floor.

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 management (especially

involvement of top-management).

Juran also emphasised the importance of education and the

cost of quality.

History part IV: Problem solving tools

7 problem solving tools (‘Magnificent Seven’)

fishbone/Ishikawa diagram

Pareto chart

run chart

defect concentration diagram

Box-and Whisker plot

...

History: K. Ishikawa

Ishikawa was very important to the quality revolution in Japan

because of his many activities (consulting, member of various

committees, ...).

Ishikawa stressed that quality control must try to meet the

requirements of customers.

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

Process thinking

all deviations from product specifications have causes in production

process

outcome production process is caused by many process factors

Methods/adjustments

People

Environment

ToolsMachine

Materials

deviation

Design of Experiments

•preferably design phase (cf. EVOP)

•off-line

•team

•detect causes of variation

•optimise process

History: G.E.P. Box

• Introduced DOE in industrial environment since early 1950’s

• Developed response surface methods at Imperial Chemical Industries

• Moved to US in 1960 and initiated Center for Quality and Productivity

Improvement in Wisconsin

• ASQC history: http://www.asq.org/join/about/history/box.html

• George Box award: http://www.enbis.org/barcelonaconference/gepbox.html

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”.

Conformance to specifications

why is product just outside specifications worse than product just

inside specifications?

classical: yes/no

LSL USL LSL USL

modern: Taguchi loss function

Conformance to specifications II

specifications of characteristic 1

spec

ific

atio

ns o

f ch

arac

teri

stic

2

History: US rediscovers quality control

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

leader in quality and became an economic superpower

Several American industries could not compete with 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 Motorola (Six Sigma).

ASQ: American Society for Quality

History: Europe

Interest in quality control in Europe did not start until the

1990’s.

Much emphasis in Europe on ISO 9000 certification.

European quality control initiatives like EFQM focus mainly

on management issues.

In 2000, ENBIS (European Network for Business and Industrial

Statistics) was founded. Initiative by Bisgaard, then

successor of Box in Wisconsin.

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 within tolerances

PROCESS output

specifications

input

History: new developments

An important new development is the Six Sigma approach

developed by Motorola (one of the first winner of the Baldrige

Award)

Six Sigma approach uses known statistical and management

concepts to come to an overall approach to quality:

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

relations between input and output parameters of processes

– use control charts on inputs instead of outputs

– direct link of efforts with reduction of costs

History part VI: Quality by design

Actions in design phase:

– prevent quality problems in production phase

– prevent quality problems when customer uses product

Typical tools: Design of Experiments, Robust Design

Corresponding definition of quality:

– quality is conformance to customer requirements and

expectations

The main 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

Summary of historical development

inspection

prevention