Chapter 10 Building the Regression Model II: Diagnostics

Transcription

1 Chapter 10 Building the Regression Model II: Diagnostics 許湘伶 Applied Linear Regression Models (Kutner, Nachtsheim, Neter, Li) hsuhl (NUK) LR Chap 10 1 / 41

2 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots a number of refined( 推敲 ) diagnostics for checking the adequacy of a regression model detecting improper functional form for a predictor variable outliers( 離群值 ) influential( 有影響的 ) observations multicollinearity hsuhl (NUK) LR Chap 10 2 / 41

3 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots Previous Chap. 3,6: check whether a curvature effect for that variable is required in the model the residual plots vs. the predictor variables: determine whether it would be helpful to add one or more of these variables to the model Figure : (Chap. 3)Residual Plots for Possible Omission of Important Predictor Variable-Productivity Example. hsuhl (NUK) LR Chap 10 3 / 41

4 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots (cont.) Limitation: not properly show the nature of the marginal effect of a predictor variable, given the other predictor variables in the model Added-variable plots(partial regression plots; adjusted variable plots;): provide graphic information about the marginal importance of a predictor variable X k, given the other predictor variables already in the model In an added-variable plot, both Y and X k under consideration are regressed against the other predictor variables and residuals are obtained for each. hsuhl (NUK) LR Chap 10 4 / 41

5 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots (cont.) The plot of these residuals: 1 the marginal importance of this variable in reducing the residual variability 2 provide information about the nature of the marginal regression relation for X k under consideration for possible inclusion in the regression model hsuhl (NUK) LR Chap 10 5 / 41

6 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots (cont.) Illustration: the regression effect for X 1, given that X 2 is already in the model (Chap. 7, p partial determination) Ŷ i (X 2 ) = b 0 + b 2 X i2 e i (Y X 2 ) = Y i Ŷ i (X 2 ) regress X 1 on X 2 : ˆX i1 (X 2 ) = b 0 + b 2X i2 e i (X 1 X 2 ) = X i1 ˆX i1 (X 2 ) ( R 2 = R 2 Y 1 2 : e i(y X 2 ) is regressed on e i (X 1 X 2 )) added-variable for predictor variable X 1 : Plot e(y X 2 ) vs e(x 1 X 2 ) hsuhl (NUK) LR Chap 10 6 / 41

7 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots (cont.) Figure : Prototype Added-Variable Plots (a) X 1 contains no additional information; no helpful to add X 1 (b) a linear band with a nonzero slope; X 1 may be a helpful addition to the regression model already containing X 2 (c) a curvilinear band; X 1 may be helpful and suggesting the possible nature of the curvature effect hsuhl (NUK) LR Chap 10 7 / 41

8 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots (cont.) Added-variable plots: providing information about the possible nature of the marginal relationship for a predictor variable, given the other variables already in the regression model the strength of the relationship useful for uncovering( 發現 ) outlying data points that may have a strong influence in estimating the relationship of the predictor variable X k to the response variable hsuhl (NUK) LR Chap 10 8 / 41

9 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots (cont.) (a) SSE(X 2 ) (b) SSE(X1, X2) Difference (SSE(X 2 ) SSE(X 1, X 2 )): SSR(X 1 X 2 ); provides information about the marginal strength of the linear relation of X 1 to the response variable, given that X 2 is in the model hsuhl (NUK) LR Chap 10 9 / 41

10 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots (cont.) Table 10.1 Average Annual Income Risk Aversion Amount of Life Insurance Carried Manager (thousand dollars) Score (thousand dollars) i X i1 X i2 Y i r 12 = Ŷ = X X 2 Residual plot: a linear relation for X 1 is not appropriate in the model already containing X 2 hsuhl (NUK) LR Chap / 41

11 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots (cont.) ex<-read.table("ch10ta01.txt",header=f) colnames(ex)<-c("x1","x2","y") attach(ex) fit<-lm(y~x1+x2) par(mfrow=c(1,3)) plot(x1,resid(fit),pch=16) abline(0,0,lty=2,col="gray") ## Method 1: plot(resid(lm(y~x2)) ~ resid(lm(x1~x2)),col="blue",pch=16, xlab="e(x_1 X_2)", ylab="e(y X_2)") abline(lm(resid(lm(y~x2))~resid(lm(x1~x2))),col="red") ## Method 2: using avplot() library(car) avplot( model=lm( Y~X1+X2 ), variable=x1 ) hsuhl (NUK) LR Chap / 41

12 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots (cont.) Ŷ (X 2 ) = X 2 ; ˆX 1 (X 2 ) = X 2 Plots: through (0,0); b 1 = ; suggest the curvilinear relation between Y and X 1 X 2 is strongly positive; a slight concave upward shape R 2 Y 1 2 = Incorporating a curvilinear effect for X 1 hsuhl (NUK) LR Chap / 41

13 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots (cont.) Comments: An added-variable plot only suggests the nature of the functional relation in which a predictor variable should be added to the regression model but does not provide an analytic expression of the relation. Added-variable plots need to be used with caution for identifying the nature of the marginal effect of a predictor variable. may not show the proper form of the marginal effect of a predictor variable if the functional relations for some or all of the predictor variables already in the regression model are misspecified the relations of the predictor variable to the response variable are complex high multicollinearity among the predictor variables hsuhl (NUK) LR Chap / 41

14 10.1 Model Adequacy for a Predictor Variable-Added Variable Plots Added Variable Plots (cont.) Any fitted multiple regression function can be obtained from a sequence of fitted partial regressions. Ex: having e(y X 2 ), e(x 1 X 2 ) e(ŷ X 2) = [e(X 1 X 2 )] [Ŷ Ŷ (X 2 )] = [X 1 ˆX 1 (X 2 )] Ŷ = X X 2 (Ŷ (X 2 ) = X 2 ; ˆX 1 (X 2 ) = X 2 ) hsuhl (NUK) LR Chap / 41

15 10.2 Identifying Outlying Y Observations- Studentized Deleted Residuals Identifying Outlying Y Observations Outlying or Extreme: the observations for these cases are well separated from the remainder of the data large residuals; have dramatic effects hsuhl (NUK) LR Chap / 41

16 10.2 Identifying Outlying Y Observations- Studentized Deleted Residuals Identifying Outlying Y Observations (cont.) Outlying: Y value, X values or both Not all outlying cases have a strong influence on the fitted regression function. A basic step: determine if the regression model under consideration is heavily influenced by one or a few cases in the data set hsuhl (NUK) LR Chap / 41

17 10.2 Identifying Outlying Y Observations- Studentized Deleted Residuals Identifying Outlying Y Observations (cont.) Two refined measures for identifying cases with outlying Y observations: Residuals, Semistudentized Residuals: e i = Y i Ŷi; e i = e i MSE Hat matrix: H = X(X X) 1 X Ŷ = HY e = (I H)Y σ 2 {e} = σ 2 (I H) σ 2 {e i } = σ 2 (1 h ii ), h ii : the ith elements on the diagonal of H σ{e i, e j } = h ij σ 2, i j s 2 {e i } = MSE(1 h ii ), s{e i, e j } = h ij MSE hsuhl (NUK) LR Chap / 41

18 10.2 Identifying Outlying Y Observations- Studentized Deleted Residuals Identifying Outlying Y Observations (cont.) h ii = X i(x X) 1 X i, X i = [1 X i,1 X i,p 1 ] Small data set: n = 4 Ŷ = X X 2 MSE = s 2 {e 1 } = 574.9( ) = hsuhl (NUK) LR Chap / 41

19 10.2 Identifying Outlying Y Observations- Studentized Deleted Residuals Identifying Outlying Y Observations (cont.) Deleted Residuals: The difference between Y i and Ŷi(i): (PRESS prediction error) delted residual: d i = Y i Ŷi(i) = e i 1 h ii h ii : the larger will be the deleted residuals as compared to the ordinary residual the estimated variance of d i : s 2 {d i } = MSE (i) (1 + X i(x (i)x (i) ) 1 X i ) = MSE (i) 1 h ii d i t((n 1) p) s{d i } hsuhl (NUK) LR Chap / 41

20 10.2 Identifying Outlying Y Observations- Studentized Deleted Residuals Identifying Outlying Y Observations (cont.) Studentized Deleted Residuals: t i = ( d i s{d i } = e i MSE(i) (1 h ii ) (n p)mse = (n p 1)MSE (i) + e2 i 1 h ii [ ] 1/2 n p 1 t i = e i SSE(1 h ii ) ei 2 h ii : the larger will be the deleted residuals as compared to the ordinary residual ) hsuhl (NUK) LR Chap / 41

21 10.2 Identifying Outlying Y Observations- Studentized Deleted Residuals Identifying Outlying Y Observations (cont.) the estimated variance of d i : s 2 {d i } = MSE (i) (1 + X i(x (i)x (i) ) 1 X i ) = MSE (i) 1 h ii d i t((n 1) p) s{d i } Test for Outliers: whose studentized deleted residuals are large in absolute value If the regression model is appropriate, so that no case is outlying. Each t i t(n p 1). t i : the appropriate Bonferroni critical value: t(1 α/2n; n p 1) hsuhl (NUK) LR Chap / 41

22 10.2 Identifying Outlying Y Observations- Studentized Deleted Residuals Identifying Outlying Y Observations (cont.) Body fat example t 13 < = t(1 α/2n; n p 1) (α =.10) The Bonferroni procedure provides a conservative( 保守的 ) test for the presence of an outlier. hsuhl (NUK) LR Chap / 41

23 Identifying Outlying X Observations-Hat Matrix Leverage Values Identifying Outlying X Observations Using H for identifying outlying X 0 h ii 1 ni=1 h ii = p h ii : called leverage; measure the distance between X i and X large h ii X i distant from the center of all Xs hsuhl (NUK) LR Chap / 41

24 Identifying Outlying X Observations-Hat Matrix Leverage Values Identifying Outlying X Observations (cont.) If X i is outlying has a large leverage h ii Ŷ i : a linear combination of Y (Ŷ = HY ) h ii : the weight of Y i the larger is h ii, the more important is Y i in determining Ŷi The larger is h ii, the smaller is σ 2 {e i }. h ii = 1 σ 2 {e i } = 0 Rule: 1 2 hii h ii > 2 h = 2 n = 2 p n ( 適用 : 2p n 1) { very high leverage:hii > 0.5 moderate leverage:h ii : hsuhl (NUK) LR Chap / 41

25 Identifying Outlying X Observations-Hat Matrix Leverage Values Identifying Outlying X Observations (cont.) Body fat example 2p/n = 0.30 h 33 = 0.372; h 15,15 = hsuhl (NUK) LR Chap / 41

26 Identifying Outlying X Observations-Hat Matrix Leverage Values Identifying Outlying X Observations (cont.) ex<-read.table("ch07ta01.txt",header=f) colnames(ex)<-c("x1","x2","x3","y") attach(ex) fit<-lm(y~x1+x2+x3) n<-length(y); p = 3 plot(x2~x1, pch=16 ) text(x1+0.5, X2, labels=as.character(1:length(x1)),col="red") hii<-hatvalues(fit) index<-hii>2*p/n points( X1[index], X2[index], cex=2.0, col= blue ) hsuhl (NUK) LR Chap / 41

27 Identifying Influential Cases-DFFITS, Cook s Distance, and DFBETAS Measures DFFITS meansure Influence on single fitted value: DFFITS-measure the influence that case i has on Ŷi (DFFITS) i = ( Ŷi Ŷ i(i) hii = t i MSE(i) h ii 1 h ii ) 1/2 Rule: { DFFITS > 1 for small to medium data sets DFFITS > 2 p/n for large data sets If X i is an outlier and has a high h ii, (DFFITS) i will tend to be large absolutely. hsuhl (NUK) LR Chap / 41

28 Identifying Influential Cases-DFFITS, Cook s Distance, and DFBETAS Measures Cook s distance Influence on all fitted value: Cook s distance-consider the influence of the ith case on all n fitted values nj=1 (Ŷ i Ŷ j(i) ) 2 D i = = (Ŷ Ŷ (i)) (Ŷ Ŷ (i)) pmse pmse [ ] = e2 i h ii pmse (1 h ii ) 2 1 the size of e i 2 the leverage h ii 3 e i or h ii D i Rule: D i F (p, n p) { little influence : P(F (p, n p) Di ) > 0.1 or 0.2 major influence : P(F (p, n p) D i ) > 0.5 hsuhl (NUK) LR Chap / 41

29 Identifying Influential Cases-DFFITS, Cook s Distance, and DFBETAS Measures Cook s distance (cont.) hsuhl (NUK) LR Chap / 41

30 Identifying Influential Cases-DFFITS, Cook s Distance, and DFBETAS Measures DFBETAS Influence on regression coefficients: DFBETAS-the difference between b k and b k(i) Rule: (DFBETAS) k(i) = b k b k(i) MSE(i) c kk, k = 0, 1,..., p 1 where c kk : the kth diagonal element of (X X) 1 { DFBETAS > 1 for small to medium data sets DFBETAS > 2 n for large data sets hsuhl (NUK) LR Chap / 41

31 Identifying Influential Cases-DFFITS, Cook s Distance, and DFBETAS Measures DFBETAS (cont.) # Body fat example (Table 10.4) > influence.measures(fit) Influence measures of lm(formula = Y ~ X1 + X2) : dfb.1_ dfb.x1 dfb.x2 dffit cov.r cook.d hat inf e e e e e e e e e e e e e e e * e e e e e e e e e e * e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e * e e e e e e e e e e e e e e e e e e e e e e e e e hsuhl (NUK) LR Chap / 41

32 Identifying Influential Cases-DFFITS, Cook s Distance, and DFBETAS Measures DFBETAS (cont.) Some final comments: Analysis of outlying and influential cases: a necessary component of good regression analysis neither automatic nor foolproof( 極簡單的 ) require good judgment by the analyst Methods described: ineffective Extensions of the single-case diagnostic procedures: computational requirements hsuhl (NUK) LR Chap / 41

33 Multicollinearity Diagnostics-VIF Variance Inflation( 膨脹 ) Factor-VIF Some problems: multicollinearity Adding or deleting X: change the regression coefficients Extra sum of squares: depending upon which other X k s are already included in the model X k highly correlated with each other s{b k } the estimated regression coefficients individually may not be statistically significant hsuhl (NUK) LR Chap / 41

34 Multicollinearity Diagnostics-VIF Variance Inflation( 膨脹 ) Factor-VIF (cont.) informal diagnostics for multicollinearity: 1 Large changes in b k when X k is added or deleted, or when an observation is altered( 改變 ) or deleted 2 Nonsignificant results in individual tests on the regression coefficients for important predictor variables. 3 Estimated regression coefficients with an algebraic sign that is the opposite of that expected from theoretical consideration or prior experience. 4 large r XX 5 Wide confidence interval for β k hsuhl (NUK) LR Chap / 41

35 Multicollinearity Diagnostics-VIF Variance Inflation( 膨脹 ) Factor-VIF (cont.) Important limitations: do not provide quantitative measurements may not identify the nature of the multicollinearity sometimes the observed behavior may occur without multicollinearity being present hsuhl (NUK) LR Chap / 41

36 Multicollinearity Diagnostics-VIF Variance Inflation( 膨脹 ) Factor-VIF (cont.) Variance Inflation Factor (VIF)( 變異數膨脹因子 ) a formal method: detecting multicollinearity; widely accepted measure how much the variances of b k s are inflated( 膨脹 ) as compared to when the predictor variables are not linearly related. Illustration: Variance-covarianve matrix of b: σ 2 {b} = σ 2 (X X) 1 hsuhl (NUK) LR Chap / 41

37 Multicollinearity Diagnostics-VIF Variance Inflation( 膨脹 ) Factor-VIF (cont.) Using the standardized regression model: Variance-covarianve matrix of b : σ 2 {b } = (σ ) 2 r 1 XX (σ ) 2 : the errir term variance for the transformed model (VIF ) k : the kth diagonal element of r 1 XX σ 2 {b k} = (σ ) 2 (VIF ) k = (σ ) 2 1 R 2 k VIF for b k: (VIF ) k = (1 R 2 k ) 1, k = 1, 2,..., p 1 R 2 k : X k is regressed on the p 2 other X k s R 2 k = 0 (VIF ) k = 1: X k is not linearly related to X k s R 2 k 0 (VIF ) k > 1: indicate inflated variance for b k as a result of the intercorrelations among the X variables hsuhl (NUK) LR Chap / 41

38 Multicollinearity Diagnostics-VIF Variance Inflation( 膨脹 ) Factor-VIF (cont.) Perfect linear association with X k s R 2 k = 1 (VIF ) k and σ 2 {b k} are unbounded Rule: largest VIF value among all Xs as an indicator of the severity( 嚴重 ) of multicollinearity max{vif 1,..., VIF p 1 } > 10 hsuhl (NUK) LR Chap / 41

39 Multicollinearity Diagnostics-VIF Variance Inflation( 膨脹 ) Factor-VIF (cont.) If VIF > 1 serious multicollinearity problems p 1 E (b k βk) 2 = (σ ) 2 k=1 p (VIF ) k k=1 large VIF larger differences between b k and β k If no linearly R 2 k 0 (VIF ) k = 1 p 1 E (b k βk) 2 = (σ ) 2 (p 1) k=1 VIF = (σ ) 2 p k=1 (VIF ) k (σ )(p 1) = p k=1 (VIF ) k (p 1) hsuhl (NUK) LR Chap / 41

40 Multicollinearity Diagnostics-VIF Variance Inflation( 膨脹 ) Factor-VIF (cont.) Figure : VIF-Body Fat Example with three Xs VIF 3 = 105 r 2 13 = = , r 2 23 = : not large X 3 : R 2 3 = 0.990; strong related to X 1, X 2 hsuhl (NUK) LR Chap / 41

41 Multicollinearity Diagnostics-VIF Variance Inflation( 膨脹 ) Factor-VIF (cont.) Comments Some program: using 1/(VIF ) k = 1 R 2 k < 0.01 (0.001, 0.001) Limitation: cannot distinguish between several simultaneous multicollinearities Other methods: more complex than VIF hsuhl (NUK) LR Chap / 41