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Design of experiments

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Approaches to Experimentation
What is Design of Experiments
Definition of DOE
Why DOE
History of DOE
Basic DOE Example
Factors, Levels, Responses
General Model of Process or System
Interaction, Randomization, Blocking, Replication
Experiment Design Process
Types of DOE
One factorial
Two factorial
Fractional factorial
Screening experiments
Calculation of Alias
DOE Selection Guide

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Design of experiments

  1. 1. © 2013 ICOPE - All rights reserved. Design of Experiments (DOE)
  2. 2. © 2013 ICOPE - All rights reserved. OVERVIEW o Approaches to Experimentation o What is Design of Experiments o Definition of DOE o Why DOE o History of DOE o Basic DOE Example o Factors, Levels, Responses o General Model of Process or System o Interaction, Randomization, Blocking, Replication o Experiment Design Process o Types of DOE o One factorial o Two factorial o Fractional factorial o Screening experiments o Calculation of Alias o DOE Selection Guide
  3. 3. © 2013 ICOPE - All rights reserved. WHAT WE LEARN Basics of DOE Full Factorial Design Fractional Factorial Design Screening Experiments Minitab Exercises
  4. 4. © 2013 ICOPE - All rights reserved. BASICS OF DOE
  5. 5. © 2013 ICOPE - All rights reserved. APPROACHES TO EXPERIMENTATION o Trail and Error Method o One Factor at a Time o Design of Experiments
  6. 6. © 2013 ICOPE - All rights reserved. BASICS OF DOE o What is DOE: Design of Experiment (DOE) is a powerful statistical technique for improving product/process designs and solving process / production problems DOE makes controlled changes to input variables in order to gain maximum amounts of information on cause and effect relationships with a minimum sample size When analyzing a process, experiments are often used to evaluate which process inputs have a significant impact on the process output and what the target level the inputs should be to achieve a desired result (output). Design of Experiments (DOE) is also referred to as Designed Experiments or Experimental Design
  7. 7. © 2013 ICOPE - All rights reserved. BASICS OF DOE o Why DOE: • Reduce time to design/develop new products & processes • Improve performance of existing processes • Improve reliability and performance of products • Achieve product & process robustness • Perform evaluation of materials, design alternatives, setting component & system tolerances
  8. 8. © 2013 ICOPE - All rights reserved. HISTORY OF DOE • The agricultural origins, 1918 – 1940s • R. A. Fisher & his co-workers • Profound impact on agricultural science • Factorial designs, ANOVA • The first industrial era, 1951 – late 1970s • Box & Wilson, response surfaces • Applications in the chemical & process industries • The second industrial era, late 1970s – 1990 • Quality improvement initiatives in many companies • CQI and TQM were important ideas and became management goals • Taguchi and robust parameter design, process robustness • The modern era, economic competitiveness and globalization is driving all sectors of the economy to be more competitive
  9. 9. © 2013 ICOPE - All rights reserved. DOE EXAMPLE
  10. 10. © 2013 ICOPE - All rights reserved. FACTORS, LEVELS, RESPONSE o Factors: Factors are inputs to the process Factors can be classified as either controllable or uncontrollable variables. In this case, the controllable factors are Flour, Eggs, Sugar and Oven. Potential factors can be categorized using the Cause & Effect Diagram o Levels: Levels represent settings of each factor in the study Examples include the oven temperature setting, no. of spoons of sugar, no. of cups of flour, and no. of eggs o Response: Response is output of the experiment In the case of cake baking, the taste, consistency, and appearance of the cake are measurable outcomes potentially influenced by the factors and their respective levels.
  11. 11. © 2013 ICOPE - All rights reserved. GENERAL MODEAL OF A PROCESS OR SYSTEM
  12. 12. © 2013 ICOPE - All rights reserved. KEY TERMINOLOGY o Interaction o Randomization o Blocking o Replication
  13. 13. © 2013 ICOPE - All rights reserved. KEY TERMINOLOGY o Interaction: Sometimes factors do not behave the same when they are looked at together as when they are alone; this is called an interaction Interaction plot can be used to visualize possible interactions between two or more factors Parallel lines in an interaction plot indicate no interaction The greater the difference in slope between the lines, the higher the degree of interaction However, the interaction plot doesn't alert you if the interaction is statistically significant Interaction plots are most often used to visualize interactions during ANOVA or DOE
  14. 14. © 2013 ICOPE - All rights reserved. KEY TERMINOLOGY o Randomization: Randomization is a statistical tool used to minimize potential uncontrollable biases in the experiment by randomly assigning material, people, order that experimental trials are conducted, or any other factor not under the control of the experimenter When we run designed experiments, we will use experimental templates to set them up and to analyze them. We do not want to actually make the experimental runs in the order shown by the template; wherever possible, we want to randomize the experimental runs. Randomization of the run order is needed to minimize the impact of those variables outside of the experiment that we are not studying.
  15. 15. © 2013 ICOPE - All rights reserved. KEY TERMINOLOGY o Blocking: Blocking is a technique used to increase the precision of an experiment by breaking the experiment into homogeneous segments (blocks or clusters or strata) in order to control any potential block to block variability Sometime we cannot totally randomize the experimental runs. Typically this is because it will be costly or will take a long time to complete the experiment. Blocking means to run all combinations at one level before running all treatment combinations at the next level. Experimental runs within blocks must be randomized.
  16. 16. © 2013 ICOPE - All rights reserved. KEY TERMINOLOGY o Replication: Replication is making multiple experimental runs for each experiment combination. This is one approach to determining the common cause variation in the process so that we can test effects for statistical significance. Repetition of a basic experiment without changing any factor settings, allows the experimenter to estimate the experimental error (noise) in the system used to determine whether observed differences in the data are “real” or “just noise”, allows the experimenter to obtain more statistical power
  17. 17. © 2013 ICOPE - All rights reserved. EXPERIMENT DESIGN PROCESS
  18. 18. © 2013 ICOPE - All rights reserved. TYPES OF DOE o One Factorial o Full Factorials o Fractional Factorials o Screening Experiments o Plackett-Burman Designs o Taguchis Orthogonal Arrays o Response Surface Analysis
  19. 19. © 2013 ICOPE - All rights reserved. ONE FACTORIAL METHOD o One Factorial method: One factorial experiments look at only one factor having an impact on output at different factor levels. The factor can be qualitative or quantitative. In the case of qualitative factors (e.g. different suppliers, different materials, etc.), no predictions can be performed outside the tested levels, and only the effect of the factor on the response can be determined. In the case of quantitative factors (e.g. temperature, voltage, load, etc.) can be used for both effect investigation and prediction, provided that sufficient data are available.
  20. 20. © 2013 ICOPE - All rights reserved. ONE FACTORIAL METHOD o One Factorial method: In single factor experiments, ANOVA models are used to compare the mean response values at different levels of the factor. Each level of the factor is investigated to see if the response is significantly different from the response at other levels of the factor. The analysis of single factor experiments is often referred to as one-way ANOVA
  21. 21. © 2013 ICOPE - All rights reserved. ONE FACTORIAL METHOD o One Factorial method: Use One-Way ANOVA (analysis of variance) to do the following when you have one categorical factor and a continuous response: Determine whether the means of two or more groups differ. Obtain a range of values for the difference between the means for each pair of groups. Where to find this analysis in Minitab: • STATISTICS > ANOVA > One-Way ANOVA
  22. 22. © 2013 ICOPE - All rights reserved. ONE FACTORIAL METHOD, EXAMPLE Consider a BPO, where the Ops Manager wants to know if the productivity (transactions per day) of his Team is same everyday. The assumption is the productivity is same on every weekday  H0, Null Hypothesis Prod Mon = Tue = Wed = Thu = Fri Ha, Alternate Hypothesis  the productivity on at least one weekday is not same. Mon != Tue = Wed = Thu = Fri (or any day avg. productivity is not equal to at least one other day’s productivity) Weekday transactions per day Weekday transactions per day Mon 30 Thu 18 Tue 16 Fri 95 Wed 22 Mon 33 Thu 15 Tue 76 Fri 32 Wed 21 Mon 18 Thu 80 Tue 27 Fri 44 Wed 18 Mon 65 Thu 35 Tue 71 Fri 12 Wed 20 Mon 38 Thu 66 Tue 22 Fri 44 Wed 12 Mon 30 Thu 33 Tue 84 Fri 19 Wed 8 Mon 55 Thu 59 Tue 34 Fri 82 Wed 12 Mon 64 Thu 98 Tue 94 Fri 37 Wed 33 Mon 84 Thu 63 Tue 12 Fri 42 Wed 16
  23. 23. © 2013 ICOPE - All rights reserved. ONE FACTORIAL METHOD, EXAMPLE Open Minitab • Go to Stat • Go to ANOVA • Go to One-way • Enter Factors • Enter Responses
  24. 24. © 2013 ICOPE - All rights reserved. ONE FACTORIAL METHOD, EXAMPLE P Value < 0.05 So we Reject null Hypothesis We can say Day of the week is significant factor on productivity. WedTueThuMonFri 100 80 60 40 20 0 Weekday transactionsperday Boxplot of transactions per day
  25. 25. © 2013 ICOPE - All rights reserved. FULL FACTORIAL METHOD o Full Factorial method: Full factorial experiments look completely at all factors included in the experimentation. In full factorials, all of the possible combinations that are associated with the factors and their levels are studied. The effects that the main factors and all the interactions between factors are measured. If we use more than two levels for each factor, we can also study whether the effect on the response is linear or if there is curvature in the experimental region for each factor and for the interactions. Full factorial experiments can require many experimental runs if many factors at many levels are investigated.
  26. 26. © 2013 ICOPE - All rights reserved. 2 FACTORIAL METHOD o 2 Factorial method: The simplest of the two level factorial experiments is the design where two factors (say factor A and factor B) are investigated at two levels. A single replicate of this design will require four runs Consider 2 factors A & B, so there will be 4 combinations (2^2) Say, 2 levels each Hi (+1) and low(-1) So the possible combinations are illustrated in the below table: Run # A B Response 1 + + 33 2 + - 52 3 - + 16 4 - - 26
  27. 27. © 2013 ICOPE - All rights reserved. 2 FACTORIAL METHOD o 2 Factorial method: Main effect of A = Mean response at+ level – Mean response at – level = (30+50)/2 – (10+20)/2 = 40 – 15 = 25 Main effect of B = Mean response at+ level – Mean response at – level = (30+10)/2 – (50+20)/2 = 20 – 35 = -15 Run # A B Response 1 + + 30 2 + - 50 3 - + 10 4 - - 20
  28. 28. © 2013 ICOPE - All rights reserved. 2 FACTORIAL METHOD o 2 Factorial method: Interaction effect of A*B = Mean response at+ level – Mean response at -level = (30+20)/2 – (50+10)/2 = 25 – 30 = -5 Interaction is obtained by multiplying the factors involved Run # A B A*B Response 1 + + + 30 2 + - - 50 3 - + - 10 4 - - + 20
  29. 29. © 2013 ICOPE - All rights reserved. TWO FACTORIAL METHOD, EXAMPLE Go to Standard tool Bar  Move cursor to DOE  Select Plan and Create  From below popup select Create Modeling Design (as we are dealing with 2 factors for now)
  30. 30. © 2013 ICOPE - All rights reserved. TWO FACTORIAL METHOD, EXAMPLE  Enter the Name of the Response Variable (or leave as is as Response)  Select the Goal  Select Factors as 2  Enter Factor Names and Settings  Enter the No. of replicates (here it is 4)  No. of runs auto populate based on factors and replicates
  31. 31. © 2013 ICOPE - All rights reserved. TWO FACTORIAL METHOD, EXAMPLE  Modeling Design Graph would appear with below details  Experimental Goal  Design Information  Factors and Settings  Effect Estimation  Detection Ability
  32. 32. © 2013 ICOPE - All rights reserved. TWO FACTORIAL METHOD, EXAMPLE  Enter your responses in Response column (C7)
  33. 33. © 2013 ICOPE - All rights reserved. TWO FACTORIAL METHOD, EXAMPLE Go to Standard tool Bar  Move cursor to DOE  Select Analyze and Interpret  From below popup select Fit Liner Model
  34. 34. © 2013 ICOPE - All rights reserved. TWO FACTORIAL METHOD, EXAMPLE  From below popup select Minimize the response and click OK
  35. 35. © 2013 ICOPE - All rights reserved. TWO FACTORIAL METHOD, EXAMPLE  From below summary Report you can identify  signifying factors from Pareto chart  Here it is B i.e., Processors per day has more impact than no. of transactions per day on Quality scores  % of Variation  design info  Optimal factor setting
  36. 36. © 2013 ICOPE - All rights reserved. TWO FACTORIAL METHOD, EXAMPLE  From below chart we can understand the Main effect and Interaction effect  It shows, transactions per day has less significant compared to Processors per day  The AB interaction plot also nearly significant
  37. 37. © 2013 ICOPE - All rights reserved. FRACTIONAL FACTORIAL METHOD o Fractional Factorial method: Fractional factorials look at more factors with fewer runs. Using a fractional factorial involves making a major assumption - that higher order interactions (those between three or more factors) are not significant. Fractional factorial designs are derived from full factorial matrices by substituting higher order interactions with new factors. To increase the efficiency of experimentation, fractional factorials give up some power in analyzing the effects on the response. Fractional factorials will still look at the main factor effects, but they lead to compromises when looking into interaction effects. This compromise is called confounding.
  38. 38. © 2013 ICOPE - All rights reserved. SCREENING EXPERIMENTS o Screening Experiments: Screening experiments are the ultimate fractional factorial experiments. These experiments assume that all interactions, even two-way interactions, are not significant. They literally screen the factors, or variables, in the process and determine which are the critical variables that affect the process output. There are two major families of screening experiments: • Drs. Plackett and Burman developed the original family of screening experiments matrices in the 1940s. • Dr. Taguchi adapted the Plackett–Burman screening designs. He modified the Plackett–Burman design approach so that the experimenter could assume that interactions are not significant, yet could test for some two-way interactions at the same time.
  39. 39. © 2013 ICOPE - All rights reserved. SCREENING EXPERIMENTS, EXAMPLE Go to Standard tool Bar  Move cursor to DOE  Select Plan and Create  From below popup select (Screen with 6 - 15 factors) Create Screening Design (as we are dealing with 6 factors for now)
  40. 40. © 2013 ICOPE - All rights reserved. SCREENING EXPERIMENTS, EXAMPLE  Enter the Name of the Response Variable (or leave as is as Response)  Select Factors as 6  Enter Factor Names and Settings  Select the No. of runs  Click OK
  41. 41. © 2013 ICOPE - All rights reserved. SCREENING EXPERIMENTS, EXAMPLE  Modeling Design Graph would appear with below details  Experimental Goal  Design Information  Factors and Settings  Effect Estimation  Detection Ability
  42. 42. © 2013 ICOPE - All rights reserved. SCREENING EXPERIMENTS, EXAMPLE  Enter your responses in Response column (C11), Quality scores
  43. 43. © 2013 ICOPE - All rights reserved. SCREENING EXPERIMENTS, EXAMPLE Go to Standard tool Bar  Move cursor to DOE  Select Analyze and Interpret  From below popup select Fit Screening Model
  44. 44. © 2013 ICOPE - All rights reserved. SCREENING EXPERIMENTS, EXAMPLE  From below popup select Yes
  45. 45. © 2013 ICOPE - All rights reserved. SCREENING EXPERIMENTS, EXAMPLE  From below summary Report you can identify  signifying factors from Pareto chart  Here it is Shift i.e., shift has more impact on Quality scores  % of Variation  design info
  46. 46. © 2013 ICOPE - All rights reserved. SCREENING EXPERIMENTS, EXAMPLE  From below chart we can understand the Main effect  It shows, Shift of day has higher impact on Quality score  The Factors shown in gray background are statistically insignificant and can be ignored from analysis
  47. 47. © 2013 ICOPE - All rights reserved. RESPONSE SURFACE ANALYSIS o Response Surface Analysis (RSM): RSM explores the relationships between several explanatory variables and one or more response variables. The method was introduced by G. E. P. Box and K. B. Wilson in 1951. The main idea of RSM is to use a sequence of designed experiments to obtain an optimal response. Response surface analysis is an off-line optimization technique. Usually, 2 factors are studied; but 3 or more can be studied. With response surface analysis, we run a series of full factorial experiments and map the response to generate mathematical equations that describe how factors affect the response.
  48. 48. © 2013 ICOPE - All rights reserved. CALCULATION OF ALIAS o Calculation of Aliases: Aliases for a fractional factorial design can be obtained using the defining relation for the design. The defining relation for the present design is: I = ABC Multiplying both sides of the previous equation by the main effect, gives the alias effect of : I*A = A*ABC A = A2BC A = BC Note that in calculating the alias effects, any effect multiplied by remains the same (), while an effect multiplied by itself results in (). Other aliases can also be obtained: B = AC and: C = AB
  49. 49. © 2013 ICOPE - All rights reserved. SECECTION GUIDE Number of Factors Comparative Objective Screening Objective 1 1 way ANOVA or Simple Regression _ 2 - 4 2 Way ANOVA, General Liner Model Full or Fractional Factorial Design 5 or more Randomized Block Design Fractional Factorial Design or Plackett- Burman

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