k introduction to spc 2ws02 industrial statistics a. di bucchianico
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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