Statistics for Environmental Engineers

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TABLE 37.1


Ion Chromatograph Data for Nitrate


Nitrate


(mg/L)


Peak 1


Peak 2


Peak 3


Average of Replicates


Standard


Deviation


Variance


0.05


390


354


371


372


18


324


0.15


1030


1028


1051


1036


12


162


0.275


1912


1909


1914


1912


2


6


0.4


2784


2779


2678


2747


60


3577


0.8


5616


5637


5612


5622


13


180


1.4


9733


9821


9786


9780


44


1963


2.0


14213


14238


14236


14229


14


193


4.0


29138


29504


29473


29372


203


41190


7.0


53065


53701


53326


53364


320


102207


10.0


75967


78499


78422


77629


1440


2073996


20.0


166985


169620


169379


168661


1457


2122090


30.0


257119


262286


262765


260723


3131


9800774


40.0


351149


353419


355161


353243


2012


4047268

Source: Greg Zelinka, Madison Metropolitan Sewerage District, Madison, WI.


with R2= 0.999. Figure 37.2 shows the data and the fitted line. The graph shows no obvious discrepancies. The large R2 is often taken as evidence that the straight line is the correct calibration curve. This is wrong. A large R2 proves nothing about whether the model adequately fits the data. This straight-line model does not fit the data.

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