Design of Engineering Experiments Chapter 5 Introduction to Factorials

Transcription

1 Design of Engineering Experiments Chapter 5 Introduction to Factorials Text reference, Chapter 5 page 170 General principles of factorial experiments The two-factor factorial with fixed effects The ANOVA for factorials Extensions to more than two factors Quantitative and qualitative factors response curves and surfaces Montgomery_chap_5 Steve Brainerd 1

2 Factorial Designs A factorial design is one in which two or more factors are investigated at all possible combinations for their levels as: #Level #factors = Total Number of experiments We will discuss 2 level designs as : 2 2 or 2 4 or 2 k In Chapter 6 FACTORS # Experments: 2 level full Factoria l= #level^#factor # Experments: 3 level full Factorial # Experments: 4 level full Factorial Montgomery_chap_5 Steve Brainerd 2

3 Factorial Designs Definitions If there are a levels of Factor A and b levels of Factor B, a Full Factorial design is one in all ab combinations are tested. Montgomery_chap_5 Steve Brainerd 3

4 Factorial Designs Definitions If there are a levels of Factor A and b levels of Factor B, a full factorial design is one in all ab combinations are tested. When factors are arranged in a factorial design, they are often called crossed. The effect of a factor is defined to be the change in the response Y for a change in the level of that factor. This is called a main effect, because it refers to the primary factors of interest in the experiment. Montgomery_chap_5 Steve Brainerd 4

5 Some Basic Definitions Definition of a factor effect: The change in the mean response when the factor is changed from low to high A= y + y = = 21 A A B = y + y = = B B AB = = Montgomery_chap_5 Steve Brainerd 5

6 The Case of Interaction: A= y + y = = A A B= y + y = = 9 B B AB = = Montgomery_chap_5 Steve Brainerd 6

7 Regression Model & The Associated Response Surface y = β + β x + β x β xx + ε The least squares fit is yˆ = x + 5.5x + 0.5xx x + 5.5x 1 2 Montgomery_chap_5 Steve Brainerd 7

8 The Effect of Interaction on the Response Surface Suppose that we add an interaction term to the model: yˆ = x + 5.5x + 8xx Interaction is actually a form of curvature Montgomery_chap_5 Steve Brainerd 8

9 Example 5-1 The Battery Life Experiment Text reference pg A = Material type; B = Temperature (A quantitative variable) 1. What effects do material type & temperature have on life? 2. Is there a choice of material that would give long life regardless of temperature (a robust product)? Montgomery_chap_5 Steve Brainerd 9

10 The General Two-Factor Factorial Experiment a levels of factor A; b levels of factor B; n replicates This is a completely randomized design Montgomery_chap_5 Steve Brainerd 10

11 Statistical (effects) model: i = 1,2,..., a yijk = µ + τi + β j + ( τβ) ij + εijk j = 1, 2,..., b k = 1,2,..., n Other models (means model, regression models) can be useful Montgomery_chap_5 Steve Brainerd 11

12 Extension of the ANOVA to Factorials (Fixed Effects Case) pg. 177 a b n a b ( yijk y... ) = bn ( yi.. y... ) + an ( y. j. y... ) i= 1 j= 1 k= 1 i= 1 j= 1 breakdown: a b a b n 2 2 ( ij. i... j....) ( ijk ij. ) i= 1 j= 1 i= 1 j= 1 k= 1 + n y y y + y + y y SS = SS + SS + SS + SS df T A B AB E abn 1= a 1+ b 1 + ( a 1)( b 1) + ab( n 1) Montgomery_chap_5 Steve Brainerd 12

13 ANOVA Table Fixed Effects Case Design-Expert will perform the computations Text gives details of manual computing (ugh!) see pp. 180 & 181 Montgomery_chap_5 Steve Brainerd 13

14 Design-Expert Output Example 5-1 Response: Life ANOVA for Selected Factorial Model Analysis of variance table [Partial sum of squares] Sum of Mean F Source Squares DF Square Value Prob > F Model < A B < AB Pure E C Total Std. Dev R-Squared Mean Adj R-Squared C.V Pred R-Squared PRESS Adeq Precision Montgomery_chap_5 Steve Brainerd 14

15 Residual Analysis Example 5-1 DESIGN-EXPERT Plot Life Normal plot of residuals DESIGN-EXPERT Plot Life Residuals vs. Predicted Normal % probability Residuals Residual Predicted Montgomery_chap_5 Steve Brainerd 15

16 Residual Analysis Example 5-1 DESIGN-EXPERT Plot Life Residuals vs. Run Residuals Run Number Montgomery_chap_5 Steve Brainerd 16

17 Residual Analysis Example 5-1 DESIGN-EXPERT Plot Life Residuals vs. Material DESIGN-EXPERT Plot Life Residuals vs. Temperature Residuals Residuals Material Temperature Montgomery_chap_5 Steve Brainerd 17

18 Interaction Plot DESIGN-EXPERT Plot Life X = B: Temperature Y = A: Material 188 Interaction Graph A: Material A1 A1 A2 A2 A3 A3 146 Life B: Temperature Montgomery_chap_5 Steve Brainerd 18

19 Quantitative and Qualitative Factors The basic ANOVA procedure treats every factor as if it were qualitative Sometimes an experiment will involve both quantitative (temperature) and qualitative (Material type) factors, such as in Example 5-1 This can be accounted for in the analysis to produce regression models for the quantitative factors at each level (or combination of levels) of the qualitative factors These response curves and/or response surfaces are often a considerable aid in practical interpretation of the results Montgomery_chap_5 Steve Brainerd 19

20 Quantitative and Qualitative Factors Response:Life *** WARNING: The Cubic Model is Aliased! *** Sequential Model Sum of Squares Sum of Mean F Source Squares DF Square Value Prob > F Mean 4.009E E+005 Linear < Suggested 2FI Quadratic Cubic Aliased Residual Total 4.785E "Sequential Model Sum of Squares": Select the highest order polynomial where the additional terms are significant. Montgomery_chap_5 Steve Brainerd 20

21 Quantitative and Qualitative Factors A = Material type B = Linear effect of Temperature B 2 = Quadratic effect of Temperature AB = Material type Temp Linear AB 2 = Material type - Temp Quad B 3 = Cubic effect of Temperature (Aliased) Candidate model terms from Design- Expert: Intercept A B B 2 AB B 3 AB 2 Montgomery_chap_5 Steve Brainerd 21

22 Quantitative and Qualitative Factors Lack of Fit Tests Sum of Mean F Source Squares DF Square Value Prob > F Linear Suggested 2FI Quadratic Cubic Aliased Pure Error "Lack of Fit Tests": Want the selected model to have insignificant lack-of-fit. Montgomery_chap_5 Steve Brainerd 22

23 Quantitative and Qualitative Factors Model Summary Statistics Std. Adjusted Predicted Source Dev. R-Squared R-Squared R-Squared PRESS Linear Suggested 2FI Quadratic Cubic Aliased "Model Summary Statistics": Focus on the model maximizing the "Adjusted R-Squared" and the "Predicted R-Squared". Montgomery_chap_5 Steve Brainerd 23

24 Quantitative and Qualitative Factors Response: Life ANOVA for Response Surface Reduced Cubic Model Analysis of variance table [Partial sum of squares] Sum of Mean F Source Squares DF Square Value Prob > F Model < A B < B AB AB Pure E C Total Std. Dev R-Squared Mean Adj R-Squared C.V Pred R-Squared PRESS Adeq Precision Montgomery_chap_5 Steve Brainerd 24

25 Regression Model Summary of Results Final Equation in Terms of Actual Factors: Material A1 Life = * Temperature * Temperature 2 Material A2 Life = * Temperature E-003 * Temperature 2 Material A3 Life = * Temperature * Temperature 2 Montgomery_chap_5 Steve Brainerd 25

26 Regression Model Summary of Results DESIGN-EXPERT Plot Life X = B: Temperature Y = A: Material 188 Interaction Graph A: Material A1 A1 A2 A2 A3 A3 146 Life B: Temperature Montgomery_chap_5 Steve Brainerd 26

27 Factorials with More Than Two Factors Basic procedure is similar to the two-factor case; all abc kn treatment combinations are run in random order ANOVA identity is also similar: SS = SS + SS + L+ SS + SS + L T A B AB AC + SS + L+ SS + SS ABC ABLK E Complete three-factor example in text, Section 5-4 Montgomery_chap_5 Steve Brainerd 27