Statistics for Environmental Engineers

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The computation of the average and the effects was explained in Chapters 27 and 28. Duplicate tests that are not reported here indicated that the variance tended to be proportional to the permeability. Because of this, a log transformation was used to stabilize the variance. The average permeability is 5.92 on the log scale. The estimated main effects (confounded with four-factor interactions) and two-factor interactions (confounded with three-factor interactions) are given in Table 29.5. These are also on the log scale.


In the absence of an estimate of a2, the significance of these effects was judged by making a normal plot, as shown in Figure 29.2. If the effects arise only from random measurement error, they will fall along


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