STATISTICAL METHODS FOR QUALITY IMPROVEMENT
1. Statistical Process Control -- Detect and reduce variation in the process.
2. Design of Experiments – Identify key variables influencing the quality characteristic of interest in
3. Acceptance Sampling – Determine whether or not products are of acceptable quality based on
inspection of a sample of units.
4. Reliability Engineering – Determine the long-term quality of products.
HISTORY OF MODERN QUALITY
- reported to manufacturing
- inspection during production
- conflict with manufacturing
- 1924 Shewart developed ―Statistical Control Chart‖ concept
- 1930 Dodge & Roming developed
―Acceptance Sampling Methods‖
2. Quality Control
- 1940’s inspection groups evolved into QC departments
- independent than manufacturing
- World War II – military products to be defect free,
- high production rate
- QC efforts died in US after the war
- 1950 Deming started his teachings in Japan
- 1960 Taguchi introduced off-line quality control methods
3. Quality Assurance
- focus on assuring process and product quality
- technical analysis
- advise on operational areas
4. Total Quality Management
- company wide quality
- 1980 Deming and Taguchi recognized in US
QUALITY of a product can be evaluated in many ways:
Performance -- will the product do the intended job?
Reliability – how often does the product fail?
Durability – how long does the product last?
Serviceability – how easy it is to repair the product?
Aesthetics – how does the product look like?
Features – what does the product do?
Perceived Quality – what is the reputation of the company?
Conformance to Standards – is the product made exactly as the designer intended?
DIFFERENT VIEWS OF QUALITY
Customer’s View (more subjective view):
Quality of the design (look, feel, function)
Consider both feature and performance measures to asses value
- Value = quality / price (determined by individual customers)
Producer’s View (more objective view):
Costs of quality (prevention, appraisal, scrap& warranty costs)
- Prevention costs: training, writing quality procedures
- Appraisal costs: inspecting and measuring product characteristics
- Scrap and rework costs: internal costs of defective products
- Warranty costs: external costs for product failures in the field
Increasing quality conformance reduces product costs and raises profits.
SOME BASIC TENETS OF MODERN VIEW OF QUALITY
1. The customer determines the quality.
2. Improving the quality requires the establishment of effective quality metrics. We must speak with data
not just opinions.
3. People working within systems create quality.
4. Quality is a moving target. It requires a commitment toward sustained continuous improvement.
5. Prevention not detection is the key to producing high quality. We must design quality into products and
6. Top management must provide leadership and support for all quality circles.
INNOVATORS OF MODERN QUALITY THINKING
U.S. Quality Innovators and the Main Years of their Work:
Walter Shewart (1920s – 1940s)
W. Edwards Deming (post WWII through 1980s)
Joseph M. Juran (consultant post WWII through 1980s)
Philip Crosby (1980s)
Armand Feigenbaum (1970s – 1980s)
Japanese Quality Innovators:
Kaora Ishikawa (post WWII – 1980s)
Genichi Taguchi (1960s – 1980s)
Shigeo Shingo (post WWII – 1980s)
WALTER A. SHEWART - Father of Modern Quality Control
“Quality is inversely proportional to variability.”
“Quality improvement is the reduction of variability in processes and products.”
Perhaps the first to successfully integrate statistics, engineering, and economics.
Developed the control chart in early 1920 while working at Bell Telephone Laboratories.
In his own words, control charts are:
―the formulation of a scientific basis for securing economic control”
Focused on the ―process‖.
Recognized the need to separate variation into assignable and unassignable causes.
- Economic Control of Quality of Manufactured Product, 1931.
- Statistical Method from the Viewpoint of Quality of Control, 1939.
First to be named an honorary member of ASQ, 1947.
ASQ Shewart Medal -- is awarded for technical leadership.
“Quality is fitness for use.”
Also well known for helping improve Japanese quality.
Directed most of his work at executives and the field of quality management.
Developed the ―Juran Trilogy‖ for managing quality:
- Quality planning, quality control, and quality improvement.
Enlightened the world on the concept of the ―vital few, trivial many‖ which is the foundation of Pareto
PHILIP CROSBY (quality management)
“Quality is conformance to clearly stated requirements.”
Four absolutes of quality including:
1. quality is defined by conformance to requirements.
2. system for causing quality is prevention not appraisal.
3. performance standards is zero defects, not close enough.
4. measurement of quality is the cost of nonconformance
Stressed a systems approach to quality (all organizations must be focused on quality)
Costs of quality may be separated into costs for prevention, appraisal, and failures (scrap, warranty,
Developed concept of true and substitute quality characteristics
- True characteristics are the customer’s view
- Substitute characteristics are the producer’s view
- Degree of match between true and substitute ultimately determines customer satisfaction
Advocate of the use of the 7 quality tools
Advanced the use of quality circles (worker quality teams)
Developed the concept of Japanese Total Quality Control
- Quality first – not short term profits.
- Next process is your customer.
- Use facts and data to make presentations.
- Respect for humanity as a management philosophy – full participation
W. EDWARDS DEMING
Studied under Shewart at Bell Laboratories
Well known for helping Japanese companies apply Shewart’s statistical process control.
Main contribution is his Fourteen Points to Quality. Some key points are
- Create constancy of purpose.
- Cease dependence on mass inspection – build quality into products.
- Drive out fear and build employee thrust.
- Break down barriers between departments (create win-win situations).
- Seek long-term supplier relationships (end low cost bidding).
- Eliminate numerical goals; abolish annual rating or merit system.
- Eliminate slogans – they provide no value in terms of improving quality.
Deming’s 14 Points:
1. Create constancy of purpose for the improvement of product or services.
2. Adopt the new philosophy.
3. Cease dependence on mass inspection for quality control
4. End the practice of awarding business on the basis of price tag.
5. Improve constantly and forever the system of production and service, to improve quality and
productivity, and thus constantly decrease costs.
6. Institute more thorough, better job-related training.
7. Institute leadership.
8. Drive out fear, so that everyone may work effectively for the company.
9. Break down barriers between departments.
10. Eliminate slogans, exhortations, and targets for the work force that ask for zero defects and new levels
11. Eliminate work standards on the factory floor.
12. Remove the barriers that rob employees at all levels in the company of their right to pride of
13. Institute a vigorous program of education and self-improvement.
14. Put everybody in the organization to work to accomplish the transformation.
The Deming Chain Reaction
Quality (products with higher
quality at less cost)
(less rework, fewer Stay in
mistakes, better use of Business
GENICHI TAGUCHI (1960s – 1980s)
“Quality is the loss from function variation and harmful effects, a product causes to society.”
Quality loss function (deviation from target is a loss to society)
Promoted the use of parameter design (application of Design of experiments) or robust engineering
Taguchi’s View of Quality versus Traditional View of Quality
Problems with Traditional Quality Definitions:
- Attribute based and qualitative in nature.— good or bad
- Manufacturing rather than design based.— final stage
- Link between customer needs and production function not established.— needs/wants
Modern View of Quality:
- Provides a quantitative basis to move the quality issue to engineering design.
- Focuses on the process not the product---prevention not containment.
- Tied to productivity; promotes continuous improvement.
- Quantifies and measures the loss to society due to poor quality.
Taguchi’s View of Quality
- Loss imparted to society during product use as a result of functional variation and harmful effects.
- functional variation: deviation of product performance from the design target.
- Loss function—means of assessing the economic impact of variation.
- Goal: develop products and processes that perform on target with smallest variation that are
insensitive to environmental conditions.
- focus is on ―engineering design‖
- robust design/parameter design
- adjust control factors so that variability due to noise factors are minimized.
- design of experiments
- signal to noise ratio—measure of performance: S/N = average / standard deviation
Example: Control the part weight of a molded product
- consider two molding machine parameters where 4 test conditions examined
Example: Comparison of variation in critical dimensions for US and Japanese transmission components.
- US – about the middle 75%
- Japan – about the middle 25%
Why would the Japanese strive to make the parts to such tight tolerances?
- warranty costs
What is the Cost of the Quality?
- Prevention costs – maintaining a quality system
- Appraisal costs – maintaining a quality assurance system
- Internal failures – manufacturing losses, scrap, rework
- External failures – warranty, repair, customer, product service
What is the Cost of not having Quality?
Example – Sony: Comparison of color TVs made in both US and Japan.
- Japan – bell shaped, a small percentage outside the specifications
- US – loaf shaped, none outside the specifications : adjusted/reworked