George Box (UNC Chapel Hill Professor) – Experimental Design for Quality Improvement (1990)
Chapters
Abstract
Revolutionizing Quality Improvement: The Comprehensive Guide to Experimental Design and the Quality Movement
Dr. George Box, a renowned expert in experimental design and quality improvement, holds a prominent role in this guide. With a Ph.D. and a Doctor of Science degree in mathematical statistics from the University of London, he is a professor emeritus and director of research at the Center for Quality and Productivity Improvement at the University of Wisconsin at Madison. Dr. Box is well-known for developing techniques like response surface methodology (RSM) and evolutionary operation (EVOP). He teaches two courses: Designing Industrial Experiment and Engineers’ Key to Quality. A new course, Designing Experiments for Discovery, Improvement, and Robustness, has been added, which goes beyond basic principles. A potential third course on control is being considered, exploring the connection between SPC and engineering automatic control.
The Essence of Quality Movement and Experimental Design
The quality movement centers on continuous improvement, involving every facet of an organization. However, experimental design is not a cure-all for quality improvement but fits into the broader context of the quality movement. R.E. Fisher, a genius statistician, developed the principles of experimental design in the 1920s. The goal is to eliminate biases and confounding factors, leading to reliable conclusions.
Experimental design serves as a tool to investigate and understand the relationship between various factors and the quality of the product or service. Through this approach, manufacturers can identify the key factors impacting quality and optimize them for better results.
Response Surfaces and Contour Graphs:
Response surfaces are a graphical representation of the relationship between multiple independent variables and a single dependent variable. Contour graphs are a series of lines connecting points of equal value on a response surface. They provide a visual representation of the relationship between variables and help identify optimal conditions.
Nonlinear Effects and Screening Experiments:
Most factors have nonlinear effects, but many exhibit linear principal effects. Running a few experiments may not yield a magic formula for every problem. Center-of-cube experiments allow for overall testing of nonlinearity. Composite designs can be used to address nonlinearity along specific axes. Measuring in the wrong metric can lead to apparent nonlinear effects.
Three-Factor Experiments and Special Response Surface Designs:
Three-factor experiments involve three independent variables and are often used to create response surfaces. Special response surface designs can be used to reduce the number of experiments required to create a response surface.
Factorial Experiments: A Paradigm Shift
The introduction of factorial experiments, pioneered by Frank Yates, deputy to R.A. Fisher, marked a significant shift in experimental methodology. In contrast to the traditional one-factor-at-a-time approach, factorial experiments test all combinations of factors simultaneously. This approach unveils intricate interactions among factors, often leading to groundbreaking discoveries and a deeper understanding of complex systems.
Screening Designs and Orthogonal Arrays:
Screening designs are used in the early stages of an investigation to identify the important factors affecting a process or system. Orthogonal arrays are useful for screening experiments because they allow for the efficient estimation of the effects of multiple factors.
Data Analysis for Identifying Important Factors:
The differences between the highs and lows of the data were calculated to identify the effects of each factor.
A normal probability plot was used to identify the real effects, which deviated from a straight line.
Cuthbert Daniel’s Normal Plot:
Cuthbert Daniel’s normal plot is a graphical representation of the effects of different factors in a factorial design experiment. Each point on the plot corresponds to a factor, and its position indicates its effect on the response variable. Points that deviate significantly from the central line indicate factors with a significant effect.
Optimal Design vs. Orthogonal Designs:
Optimal designs require prior knowledge of factors, regions, and functions, which is often unavailable.
Factorial experiments are powerful tools for exploring unknown systems.
Factorial designs allow for comparisons and identification of significant differences.
Orthogonality in factorial designs facilitates the isolation and interpretation of effects.
Economic Considerations and Practical Applications
In industrial experimentation, economic considerations play a crucial role. Balancing improvement with cost entails careful consideration of counteracting factors like yield, energy usage, and product power. Techniques such as the 25% rule in budget allocation and the use of screening designs help navigate these complexities, optimizing the balance between multiple factors.
The Helicopter Experiment: A Case Study in Interactive Learning
The paper helicopter experiment exemplifies the practical application of quality improvement principles. Through brainstorming, a factorial design involving multiple variables, and hands-on experimentation, significant factors like wing length and body length were identified. This experiment serves as a model for modern education, emphasizing problem-solving and interactive learning over traditional rote memorization.
Factorial Design Experiments:
Factorial design experiments utilize multiple factors and their combinations to analyze their effects on a response variable. They include both low and high levels for each factor, resulting in a balanced design.
Calculating Factor Effects:
The average response at a low factor level is compared to the average response at a high factor level. The difference between these averages indicates the effect of that factor on the response variable.
Designing Paper Helicopters for Experimental Design Lessons:
Illustrating the concept of experimental design using the example of designing paper helicopters.
Experiment Design:
Eight factors identified as potentially influential: wing length, body length, body width, wing width, wing fold, paper clip attachment, etc.
Sixteen helicopters created using a recipe based on these factors.
Data Collection:
Experiment conducted by dropping the 16 helicopters and recording their flight times.
Results:
Analysis revealed that wing length and body length were the most significant factors affecting flight time.
Longer wings and shorter bodies resulted in longer flight times.
Additional Considerations:
Thickness of the body later identified as another important factor.
Encouragement to explore variations and experiment with different factors.
Benefits:
Experimental design provides a practical and engaging way to introduce statistical concepts to engineers.
Hands-on approach allows engineers to grasp the principles without the need for prior statistical knowledge.
Future Directions: Continuous Improvement and Beyond
The guide concludes with insights from George Box and others, emphasizing the continuous improvement philosophy. Covering topics like TRIZ, teamwork, and robust design, the guide not only presents a historical overview but also looks ahead, outlining courses and on-site training opportunities to equip professionals with cutting-edge skills in quality and experimental design.
Notes by: datagram