Statistics for Environmental Engineers

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Experimental Conditions and Responses for Eight Fly Ash Specimens


107.6


118.9


E


о


О

+


FIGURE 27.1 Cube plot showing the measured densities for the eight experimental conditions of the 23 factorial design.


Factor_ Density


Run


W (%)


C (psi)


T (min)


(lb/ft3)


1


4


60


5


107.9


2


10


60


5


120.8


3


4


260


5


118.6


4


10


260


5


126.5


5


4


60


20


99.8


6


10


60


20


117.5


7


4


260


20


107.6


8


10


260


20


118.9

reaction time. Densities are higher at the top of the cube than at the bottom, showing that higher pressure increases density. Density is lower at the back of the cube than at the front, showing that long reaction time reduces density. Higher water content increases density. The difference between the response at high and low levels is called a main effect. They can be quantified and tested for statistical significance.


It is possible that density is affected by how the factors act in combination. For example, the effect of water content at 20-min reaction time may not be the same as at 5 min. If it is not, there is said to be a two-factor interaction between water content and reaction time. Water content and compaction might interact, as might compaction and time.

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