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6166 UNIT 3 Section 1 Answers

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Problem 8.30, page 424 of Ott and Longnecker. A small corporation make insulation shields for electrical wires using three different types of machines. The corporation wants to evaluate the variation in the inside diameter dimension of the shields produced by the machines. A quality engineer at the corporation randomly selects shields produced by each of the machines and records the inside diameters of each shield (in millimeters). She wants to determine whether the means and standard deviations of the three machines differ. The data are given below.
Machine_C Machine_B Machine_A 29.7 8.7 18.1 18.7 56.8 2.4 16.5 4.4 2.7 63.7 8.3 7.5 18.9 5.8 11.0 107.2 . . 19.7 . . 93.4 . . 21.6 . . 17.8 . .
Answers:
y_{ij} = m + a_{i} + e_{ij}
2. We will use SAS Proc GLM to obtain the computations of the AOV. The SAS program is fairly long and can be found here. This program has comments describing what the purpose is of each procedure. Use the SAS help files to understand what each procedure does. The full output from the SAS program can be viewed here. Note that SAS can produce a lot of output. The AOV table is reproduced below. Note that with a pvalue (Pr>F)=0.0939 we would not reject the null hypothesis of equal machine means at the a=0.05 level.
Table 1. Analysis of variance table for the test of the hypothesis of equal machine means.
Sum of Source DF Squares Mean Square F Value Pr > F Model 2 4141.04150 2070.52075 2.73 0.0939 Error 17 12907.38800 759.25812 Corrected Total 19 17048.42950
The Means statement in the Proc GLM block has the option to perform tests for homogeneity of variances. Here I have asked it to consider two tests, Bartlett's and Brown and Forsythe' s modification of Levene's test. The results are reproduced here.
Table 2. Machine means and standard deviations with associated test statistics of the hypothesis of equal machine variances.
Level of diameter machine N Mean Std Dev a 5 8.3400000 6.5217329 b 5 16.8000000 22.4311168 c 10 40.7200000 34.5199395 Bartlett's Test for Homogeneity of diameter Variance Source DF ChiSquare Pr > ChiSq machine 2 8.3489 0.0154 Brown and Forsythe's Test for Homogeneity of diameter Variance ANOVA of Absolute Deviations from Group Medians Sum of Mean Source DF Squares Square F Value Pr > F machine 2 1144.9 572.4 0.84 0.4480 Error 17 11555.8 679.8
Bartlett's test suggests there are significant heterogeneity of variances whereas the BF test suggest the variances are the same. Note in the table the large differences between the machine A standard deviation and the machine C standard deviation. Despite the BF tests results, I might be a little uneasy about the assumption of common variance. But we reserve judgement until we look at the normality of residuals.
The statistics for testing the normality of residuals are computed in Proc Capability. The important part of this analysis is given below.
Table 3. Test statistics for assessing normality of residuals from the analyis of variance model.
Tests for Normality Test Statistic p Value ShapiroWilk W 0.811160 Pr < W 0.001 KolmogorovSmirnov D 0.235658 Pr > D <0.010 Cramervon Mises WSq 0.255007 Pr > WSq <0.005 AndersonDarling ASq 1.438729 Pr > ASq <0.005
Note that the ShapiroWilk test (and all othere tests we have not covered in this class) suggest that the residuals are not normally distributed. Histograms and normal probability plots of the residuals back up these tests.
Figure 1. Histogram of residuals with overlay of normal density curve.
Figure 2. Normal Probability Plot of residuals.
Note that the residuals do not look very normal. To determine what form of transformation might be used, we plot the machine sample means versus the machine sample variances and look for linearity.
Figure 3. Plot of sample means and variances for the three machines.
The relationship is fairly linear suggesting that a square root transformation should be used. Rerunning the analysis on the square root transformed data suggests that there are significant differences between machines in the average diameters when measured on the square root scale.
Table 4. Test statistics for square root transformed diameters.
Dependent Variable: sdia Sum of Source DF Squares Mean Square F Value Pr > F Model 2 41.4272950 20.7136475 4.48 0.0274 Error 17 78.6683337 4.6275490 Corrected Total 19 120.0956287 Level of sdia machine N Mean Std Dev a 5 2.70040161 1.14446010 b 5 3.57461248 2.24224933 c 10 5.94879442 2.43398279 Bartlett's Test for Homogeneity of sdia Variance Source DF ChiSquare Pr > ChiSq machine 2 2.2835 0.3193 Brown and Forsythe's Test for Homogeneity of sdia Variance ANOVA of Absolute Deviations from Group Medians Sum of Mean Source DF Squares Square F Value Pr > F machine 2 2.2704 1.1352 0.31 0.7364 Error 17 61.9489 3.6441 Tests for Normality Test Statistic p Value ShapiroWilk W 0.801182 Pr < W 0.001 KolmogorovSmirnov D 0.246866 Pr > D <0.010 Cramervon Mises WSq 0.278129 Pr > WSq <0.005 AndersonDarling ASq 1.593010 Pr > ASq <0.005
The tests of normality of residuals again suggest that residuals are not truely normal, but the Homogeneity of Variance tests suggest much stronger similarity in variances across machines.
Figure 4. Normal probability plot of residuals from the analysis of variance on square root transformed diameters.
There is no guarantee that any other transformation will do any better than the square root transformation. At this point we might consider performing a nonparameteric analysis of variance test to confirm or deny the existance of Machine differences in mean diameter. Since the nonparameteric test depends on ranks and not the original values, and the square root transformation does not change the ranks, we simply perform the analysis on the original data.
The nonparameteric test of choice here is the KruakalWallis procedure. Results of this analysis are in the SAS output. The important parts of this analysis are presented below.
Table 5 Test statistics associated with the KurskalWallis procedure.
KruskalWallis one way analysis of variance for diameter data for Problem 8.30 The NPAR1WAY Procedure Wilcoxon Scores (Rank Sums) for Variable diameter Classified by Variable machine Sum of Expected Std Dev Mean machine N Scores Under H0 Under H0 Score  a 5 27.0 52.50 11.456439 5.40 b 5 37.0 52.50 11.456439 7.40 c 10 146.0 105.00 13.228757 14.60 KruskalWallis Test ChiSquare 9.8914 DF 2 Pr > ChiSquare 0.0071The pvalue for the KW test (Pr > ChiSquare)=0.0071 suggests that we should reject the null hypothesis of equal machine MEDIANS and entertain the alternative hypothesis of unequal median responses. Note that the results of the KruskalWallis test are similar to that obtained with the analysis on the square root transformed responses and is different from that observed with the untransformed diameters. The importance of satisfying the homogeneity of variance and normality of residuals assumptions is illustrated in this example.