Chapter 5 Introduction to Factorial Designs 1 5.1 Basic Definitions and Principles • • • • Study the effects of two or more factors. Factorial designs Crossed: factors are arranged in a factorial design Main effect: the change in response produced by a change in the level of the factor 2 Definition of a factor effect: The change in the mean response when the factor is changed from low to high 40 52 20 30 21 2 2 30 52 20 40 B yB yB 11 2 2 52 20 30 40 AB 1 2 2 A y A y A 3 50 12 20 40 A y A y A 1 2 2 40 12 20 50 B yB yB 9 2 2 12 20 40 50 AB 29 2 2 4 Regression Model & The Associated Response Surface y 0 1 x1 2 x2 12 x1 x2 The least squares fit is yˆ 35.5 10.5 x1 5.5 x2 0.5 x1 x2 35.5 10.5 x1 5.5 x2 5 The Effect of Interaction on the Response Surface Suppose that we add an interaction term to the model: yˆ 35.5 10.5 x1 5.5 x2 8x1 x2 Interaction is actually a form of curvature 6 • When an interaction is large, the corresponding main effects have little practical meaning. • A significant interaction will often mask the significance of main effects. 7 5.2 The Advantage of Factorials • One-factor-at-a-time desgin • Compute the main effects of factors A: A+B- - A-BB: A-B- - A-B+ Total number of experiments: 6 • Interaction effects A+B-, A-B+ > A-B- => A+B+ is better??? 8 5.3 The Two-Factor Factorial Design 5.3.1 An Example • a levels for factor A, b levels for factor B and n replicates • Design a battery: the plate materials (3 levels) v.s. temperatures (3 levels), and n = 4: 32 factorial design • Two questions: – What effects do material type and temperature have on the life of the battery? – Is there a choice of material that would give uniformly long life regardless of temperature? 9 • The data for the Battery Design: 10 • Completely randomized design: a levels of factor A, b levels of factor B, n replicates 11 • Statistical (effects) model: i 1, 2,..., a yijk i j ( )ij ijk j 1, 2,..., b k 1, 2,..., n is an overall mean, i is the effect of the ith level of the row factor A, j is the effect of the jth column of column factor B and ( )ij is the interaction between i and j . • Testing hypotheses: H 0 : 1 a 0 v.s. H 1 : at least one i 0 H 0 : 1 b 0 v.s. H 1 : at least one j 0 12 H 0 : ( ) ij 0 i, j v.s. H 1 : at least one ( ) ij 0 • 5.3.2 Statistical Analysis of the Fixed Effects Model b n yi.. yijk yi.. j 1 k 1 a yi.. bn n y. j . yijk y. j . i 1 k 1 n yij. yijk yij. k 1 a b n y... yijk i 1 j 1 k 1 y. j . an yij. n y... y... abn 13 a b n a b 2 ( y y ) bn ( y y ) an ( y y ) ijk ... i.. ... . j. ... 2 i 1 j 1 k 1 2 i 1 a j 1 b a b n n ( yij . yi.. y. j . y... ) ( yijk yij . ) 2 2 i 1 j 1 i 1 j 1 k 1 SST SS A SS B SS AB SS E df breakdown: abn 1 a 1 b 1 (a 1)(b 1) ab(n 1) 14 • Mean squares a E ( MS A ) E ( SS A /(a 1)) 2 bn i2 i 1 a 1 b E ( MS B ) E ( SSB /(b 1)) 2 an j2 j 1 b 1 a b n ( ) ij2 SS AB i 1 j 1 E ( MS AB ) E ( ) 2 (a 1)(b 1) (a 1)(b 1) SSE E ( MS E ) E ( ) 2 ab(n 1) 15 • The ANOVA table: 16 17 Example 5.1 Response: Life ANOVA for Selected Factorial Model Analysis of variance table [Partial sum of squares] Source Model A B AB Pure E C Total Sum of Squares 59416.22 10683.72 39118.72 9613.78 18230.75 77646.97 DF 8 2 2 4 27 35 Mean F Square Value 7427.03 11.00 5341.86 7.91 19559.36 28.97 2403.44 3.56 675.21 Std. Dev. 25.98 Mean 105.53 C.V. 24.62 R-Squared Adj R-Squared Pred R-Squared 0.7652 0.6956 0.5826 PRESS Adeq Precision 8.178 32410.22 Prob > F < 0.0001 0.0020 < 0.0001 0.0186 18 DESIGN-EXPERT Plot Life Interaction Graph A: Material 188 X = B: Temperature Y = A: Material A1 A1 A2 A2 A3 A3 Life 146 104 2 2 62 2 20 15 70 125 B: Tem perature 19 • Multiple Comparisons: – Use the methods in Chapter 3. – Since the interaction is significant, fix the factor B at a specific level and apply Turkey’s test to the means of factor A at this level. – See Page 174 – Compare all ab cells means to determine which one differ significantly 20 5.3.3 Model Adequacy Checking • Residual analysis: eijk yijk yˆ ijk yijk yij DESIGN-EXPERT Plot Life Normal plot of residuals DESIGN-EXPERT Plot Life Residuals vs. Predicted 45.25 99 95 18.75 80 70 Res iduals Norm al % probability 90 50 30 20 10 -7.75 -34.25 5 1 -60.75 49.50 -60.75 -34.25 -7.75 18.75 76.06 102.62 129.19 155.75 45.25 Predicted Res idual 21 DESIGN-EXPERT Plot Life Residuals vs. Run 45.25 Res iduals 18.75 -7.75 -34.25 -60.75 1 6 11 16 21 26 31 36 Run Num ber 22 DESIGN-EXPERT Plot Life Residuals vs. Material Residuals vs. Temperature 45.25 45.25 18.75 18.75 Res iduals Res iduals DESIGN-EXPERT Plot Life -7.75 -7.75 -34.25 -34.25 -60.75 -60.75 1 2 Material 3 1 2 3 Tem perature 23 5.3.4 Estimating the Model Parameters • The model is yijk i j ( ) ij ijk • The normal equations: a b i 1 j 1 a b : abn bn i an j n ( ) ij y i 1 j 1 b b j 1 j 1 i : bn bn i n j n ( ) ij yi a a i 1 i 1 j : an n i an j n ( ) ij y j ( ) ij : n n i n j n( ) ij y ij • Constraints: a i 1 i b a b j 1 i 1 j 1 0, j 0, ij ij 0 24 • Estimations: ˆ y ˆi y i y ˆ j y j y ij y ij y i y j y • The fitted value: yˆ ijk ˆ ˆi ˆ j ij yij • Choice of sample size: Use OC curves to choose the proper sample size. 25 • Consider a two-factor model without interaction: – Table 5.8 – The fitted values: yˆ ijk yi y j y 26 • One observation per cell: – The error variance is not estimable because the two-factor interaction and the error can not be separated. – Assume no interaction. (Table 5.9) – Tukey (1949): assume ()ij = rij (Page 183) – Example 5.2 27 5.4 The General Factorial Design • More than two factors: a levels of factor A, b levels of factor B, c levels of factor C, …, and n replicates. • Total abc … n observations. • For a fixed effects model, test statistics for each main effect and interaction may be constructed by dividing the corresponding mean square for effect or interaction by the mean square error. 28 • Degree of freedom: – Main effect: # of levels – 1 – Interaction: the product of the # of degrees of freedom associated with the individual components of the interaction. • The three factor analysis of variance model: – yijkl i j k ( ) ij ( ) ik ( ) jk ( ) ijk ijkl – The ANOVA table (see Table 5.12) – Computing formulas for the sums of squares (see Page 186) – Example 5.3 29 30 • Example 5.3: Three factors: the percent carbonation (A), the operating pressure (B); the line speed (C) 31 32 5.5 Fitting Response Curves and Surfaces • An equation relates the response (y) to the factor (x). • Useful for interpolation. • Linear regression methods • Example 5.4 – Study how temperatures affects the battery life – Hierarchy principle 33 – Involve both quantitative and qualitative factors – 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 A = Material type B = Linear effect of Temperature B2 = Quadratic effect of Temperature AB = Material type – TempLinear AB2 = Material type - TempQuad B3 = Cubic effect of Temperature (Aliased) 34 35 36 37 5.6 Blocking in a Factorial Design • A nuisance factor: blocking • A single replicate of a complete factorial experiment is run within each block. • Model: yijk i j ( ) ij k ijk – No interaction between blocks and treatments • ANOVA table (Table 5.20) 38 39 • Example 5.6: – Two factors: ground clutter and filter type – Nuisance factor: operator 40 • Two randomization restrictions: Latin square design • An example in Page 200. • Model: yijkl i j k ( ) jk l ijkl • Tables 5.23 and 5.24 41