Statistics for Environmental Engineers

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5.6


10.2


4.7


12.0


3.7


6.2


3.9


22.7


7.9


19.8


6.3


12.6


4.4


7.9


5.4


7.1


4.5


9.2


5.2


9.5


5.4


10.1


4.1


6.4


4.2


5.9


3.8


5.7


4.0


27.5


5.7


9.4


5.2


5.7


3.9


7.5


5.9


17.2


3.7


20.5


5.6


8.1


4.9


8.0


5.7


10.0


5.2


10.7


3.1


19.1


4.1


15.7


9.8


11.1


5.4


2.8


3.1


9.5


3.7


21.3


5.1

Note: Concentrations are expressed as mg/L.


The regression is not strictly valid because both BOD and COD are subject to considerable measurement error. The regression correctly indicates the strength of a linear relation between BOD and COD, but any statements about probabilities on confidence intervals and prediction would be wrong.

Spearman Rank-Order Correlation


Sometimes, data can be expressed only as ranks. There is no numerical scale to express one’s degree of disgust to odor. Taste, appearance, and satisfaction cannot be measured numerically. Still, there are situations when we must interpret nonnumeric information available about odor, taste, appearance, or satisfaction. The challenge is to relate these intangible and incommensurate factors to other factors that can be measured, such as amount of chlorine added to drinking water for disinfection, or the amount of a masking agent used for odor control, or degree of waste treatment in a pulp mill.

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