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STATISTICAL QUALITY CONTROL
Majid Rafiee
Department of Industrial Engineering
Sharif University of Technology
rafiee@sharif.ir
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Quality Improvement in Modern Business
Environment
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Copyright Notice
• Parts (text & figures) of this lecture adopted from:
• Introduction to Statistical Quality Control, 5th Edition, by Douglas C.
Montgomery , Arizona State University
• https://www.wikipedia.org
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Outline
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Meaning of Quality and Quality Improvement
History of Quality Control
Statistical Methods for Quality Control
Management Aspects of Quality Control
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Outline
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Meaning of Quality and Quality Improvement
History of Quality Control
Statistical Methods for Quality Control
Management Aspects of Quality Control
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Are These High Quality Shoes??
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Quality Has Different Dimensions
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Performance
Reliability
Durability
Serviceability
Aesthetics
Features
Perceived Quality
Conformance to Standards
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Quality Has Different Dimensions
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Performance
Reliability
Durability
Serviceability
Aesthetics
Features
Perceived Quality
Conformance to Standards
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Performance
• Refers to a product's primary operating characteristics.
• Involves measurable attributes.
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Quality Has Different Dimensions
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Performance
Reliability
Durability
Serviceability
Aesthetics
Features
Perceived Quality
Conformance to Standards
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Reliability
• Is the likelihood that a product will not fail within a specific time
period.
• This is a key element for users who need the product to work
without fail.
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Reliability
• R(Product Name)Time = Probability
• R(Airbus)10h = 0.999999998
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Quality Has Different Dimensions
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Performance
Reliability
Durability
Serviceability
Aesthetics
Features
Perceived Quality
Conformance to Standards
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Durability
• Measures the length of a product’s life.
• When the product can be repaired, estimating durability is more
complicated.
• The item will be used until it is no longer economical to operate it.
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Quality Has Different Dimensions
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Performance
Reliability
Durability
Serviceability
Aesthetics
Features
Perceived Quality
Conformance to Standards
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Serviceability
• Is the speed with which the product can be put into service when it
breaks down.
• Consumer's ease of obtaining repair service
• Example: access to service centers
• Ease of self-service
• reliability of service
• example: whether the service is performed right the first time
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Quality Has Different Dimensions
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Performance
Reliability
Durability
Serviceability
Aesthetics
Features
Perceived Quality
Conformance to Standards
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Aesthetics or Style
• The aesthetic properties of a product contribute to the identity of a
company or a brand.
• Aesthetics is the subjective dimension indicating the kind of
response a user has to a product.
• It represents the individual’s personal preference.
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Quality Has Different Dimensions
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Performance
Reliability
Durability
Serviceability
Aesthetics
Features
Perceived Quality
Conformance to Standards
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Features
• Features are additional characteristics that enhance the appeal of
the product or service to the user.
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Quality Has Different Dimensions
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Performance
Reliability
Durability
Serviceability
Aesthetics
Features
Perceived Quality
Conformance to Standards
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Perceived Quality
• Perceived Quality is the quality attributed to a good or service based
on indirect measures.
• Consumers do not always have complete information about a product's
or service's attributes; indirect measures may be their only basis for
comparing brands.
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Made In ChinaMade In Iran
Made In Japan…
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Quality Has Different Dimensions
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Performance
Reliability
Durability
Serviceability
Aesthetics
Features
Perceived Quality
Conformance to Standards
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Conformance to Standards
• Is the precision with which the product or service meets the
specified standards.
• Target
• Tolerance
• the range of permitted deviation from the target
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What is The Meaning of Quality?
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What is The Meaning of Quality?
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• Quality means fitness to use
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What is The Meaning of Quality?
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Quality means fitness to use
This is a traditional definition
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What is The Meaning of Quality?
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Quality is inversely proportional to variability
This is a modern definition of quality
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Quality is inversely proportional to “variability”
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Quality is inversely proportional to “variability”
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• The Transmission Example
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What is The Meaning of Quality Improvement ?
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• Elimination of waste
• useful in service or transactional businesses
• Reduction of variability in processes & products
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What Is Quality Engineering?
• Variability is described in statistical terms
• Statistical methods play central role in quality improvement
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Applying statistical methods to quality
engineering.
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What Is Quality Engineering?
Quality engineering is the combination of engineering, statistics, and
management to reduce variability while maintaining the average.
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Quality Characteristics
• Also called Critical-To-Quality ( CTQ )
• What the user or consumer thinks of as quality
• Types of Quality Characteristics
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Physical Time Orientation
Sensory
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Types of Quality Characteristics
• Physical
• Length, weight, voltage, viscosity
• Sensory
• Taste, appearance, color
• Time orientation
• Reliability, durability, serviceability
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[from: https://www.latestquality.com/ctq-tree-example/, last seen on January 2019]
https://www.latestquality.com/ctq-tree-example/
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Classifying Data On Quality Characteristics
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Data
Variables Attributes
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Classifying Data On Quality Characteristics
• Variables
• Often continuous measurements
• Length, voltage, viscosity
• Following continuous distribution
• Attributes
• Usually discrete data
• Often taking the form of counts
• Following discrete distribution
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Outline
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Meaning of Quality and Quality Improvement
History of Quality Control
Statistical Methods for Quality Control
Management Aspects of Quality Control
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History
• Walter A. Shewhart
• 1920’s
• Control chart
• More on Wiki
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https://en.wikipedia.org/wiki/Walter_A._Shewhart
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Outline
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Meaning of Quality and Quality Improvement
History of Quality Control
Statistical Methods for Quality Control
Management Aspects of Quality Control
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Statistical Methods for Quality Control
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Statistical Methods for Quality Control
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Statistical Methods
Acceptance Sampling
(AS)
Designed experiments
(DOX)
Statistical process control (SPC)
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Statistical Methods for Quality Control
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Statistical Methods
Acceptance Sampling
(AS)
Designed experiments
(DOX)
Statistical process control (SPC)
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Acceptance Sampling (AS)
• Uses statistical sampling to determine whether to accept or reject a production lot of material
• Off-line technique (after production)
• Pros
• Simplest quality control technique
• Economy of time and money in comparison to 100% inspection
• Cons
• Not corrective & preventive
• likelihood or risk
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Variations Of Acceptance Sampling
• outgoing inspection
• Receiving/ incoming inspection
• Disposition of lots
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Variations Of Acceptance Sampling
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Statistical Methods for Quality Control
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Statistical Methods
Acceptance Sampling
(AS)
Designed experiments
(DOX)
Statistical process control (SPC)
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Designed experiments (DOX)
• Discovering the key factors that influence process performance for
Process optimization
• Off-line technique (before production)
• Pros
• The most desirable quality control technique
• corrective & preventive
• Cons
• Cost & time needed
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Designed experiments (DOX)
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A factorial experiment with three factors
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Statistical Methods for Quality Control
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Statistical Methods
Acceptance Sampling
(AS)
Designed experiments
(DOX)
Statistical process control (SPC)
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Statistical process control (SPC)
• Using Control charts, plus other problem-solving tools. Useful in
monitoring processes, reducing variability through elimination of
assignable causes
• On-line technique (during production)
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Control chart & Specifications
• a statistical process control tool used to determine if a
manufacturing or business process is in a state of control.
• Specifications
• Lower specification limit
• Upper specification limit
• Target or nominal values
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Control chart & Specifications
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Outline
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Meaning of Quality and Quality Improvement
History of Quality Control
Statistical Methods for Quality Control
Management Aspects of Quality Control
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Management Aspects of Quality Control
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Management Aspects of Quality Control
Quality Planning
Quality Assurance
Quality Control and
Improvement
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Management Aspects of Quality Control
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Management Aspects of Quality Control
Quality Planning
Quality Assurance
Quality Control and
Improvement
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Quality Planning
• Strategic activity
• As important as financial plan, marketing plan, …
• Involves identifying customers & their needs
• Listening to voice of the customer
• Developing products or services meet or exceed customer
expectations
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Management Aspects of Quality Control
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Management Aspects of Quality Control
Quality Planning
Quality Assurance
Quality Control and
Improvement
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Quality Assurance
• Is the set of activities that ensures the quality level
• Documentation of quality system
• Involves for: policy, procedures, work instructions and specifications
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Management Aspects of Quality Control
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Management Aspects of Quality Control
Quality Planning
Quality Assurance
Quality Control and
Improvement
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Quality Control and Improvement
• Involve the set of activities used to ensure that the products &
services meet requirements and are improved on a continuous basis
• Using statistical techniques
• SPC, DOX, …
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Quality Philosophies and Management Strategies
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Feigenbaum
Total Quality Control
quality leadership, quality technology, organizational
commitment
Deming
PDCA
Plan, do, control, act
Juran Trilogy
Planning, Control,
Improvement
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Common Concepts of Quality Philosophies
• Importance of leadership & management in quality improvement
• Emphasis on using statistical methods
• Quality as a competitive advantage
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International Organization for Standardization (ISO)
• Wiki
• is an international standard-setting body composed of representatives
from various national standards organizations.
• ISO 9000, ISO 14000, ISO/TS 16949, …
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Quality Awards
• EFQM, MBNQA, Deming Prize, …
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Six Sigma
• Focus of Six Sigma is on Process Improvement with an Emphasis on
Achieving Significant Business Impact
• Involves a five-step process (DMAIC)
• Define, Measure, Analyze, Improve, Control
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[from: http:// xlncacademy.com/blog/top-5-unique-benefits-of-six-sigma , last seen on January 2019]
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Six Sigma
• LSL & USL are fixed
• It is the process which changes
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Six Sigma
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Six Sigma
• A visit to a fast-food store: Hamburger (bun, meat, special
sauce, cheese, pickle, onion, lettuce, tomato), fries, and
drink. This product has 10 components.
• Q1) Probability of customer satisfaction for a single meal in
Six Sigma & Three Sigma?
• Q2) Family of four, once a month; Probability of customer
satisfaction for all meals in a month & in a year ?
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The Process Improvement Triad:DFSS, Lean, and DMAIC
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Design for Six Sigma (DFSS)
• Only so much improvement can be wrung out of an
existing system
• New process design
• New product design (engineering)
• DFSS Matches Customer Needs with Capability &
enhances product design methods
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Lean
• A set of methods and tools used to eliminate waste in
a process
• identify anything not absolutely required to deliver a
quality product on time
• reduce inventory, lead time, and cost
• increase productivity, quality, on time delivery,
capacity, and sales
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DMAIC
• Solves problems by using Six Sigma tools
• Define problems in processes
• Measure performance
• Analyze causes of problems
• Improve processes/ remove variations and nonvalue-
added activities
• Control processes so problems do not recur
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[from: http:// xlncacademy.com/blog/top-5-unique-benefits-of-six-sigma , last seen on January 2019]
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Six Sigma
• Pros
• A limit of 3.4 defects per one million products or services
• addresses the entire process, rather than just the final outcome
• It is proactive rather than reactive
• determines how improvements can be made even before defects are found
• Useful for product with several components
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Six Sigma
• Cons
• rigidity that can create delays & stifle creativity
• an inexpensive measure that carries a risk of a higher defect rate is
rejected in favor of a more expensive measure that helps to achieve Six
Sigma, but adversely affects profitability
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Quality Costs
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Quality Costs
Prevention Costs
Appraisal Costs
Internal Failure Costs
External Failure Costs
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Quality Costs
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Quality Costs
Prevention Costs
Appraisal Costs
Internal Failure Costs
External Failure Costs
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Prevention Costs
• Quality planning & engineering
• New products review
• Product/ process design
• Process control
• Burn-in
• Training
• Quality data acquisition & analysis
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Quality Costs
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Quality Costs
Prevention Costs
Appraisal Costs
Internal Failure Costs
External Failure Costs
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Appraisal Costs
• Inspection & test of incoming materials
• Product inspection & test
• Materials & services consumed
• Maintaining accuracy of test equipment
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Quality Costs
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Quality Costs
Prevention Costs
Appraisal Costs
Internal Failure Costs
External Failure Costs
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Internal Failure Costs
• Scrap
• Rework
• Retest
• Failure analysis
• downtime
• Yield losses
• Down grading
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Quality Costs
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Quality Costs
Prevention Costs
Appraisal Costs
Internal Failure Costs
External Failure Costs
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External Failure Costs
• Complaint adjustment
• Returned product/ material
• Warranty charges
• Liability costs
• Indirect costs
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Backup
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