Statistics for Environmental Engineers

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4 49    П 1


(yi-yj)± 72((m)10 +10


Table of tk-1,v,005/2 for k — 1 Two-Sided Comparisons for a Joint 95% Confidence Level Where There are a Total of k Treatments, One of Which is a Control


k — 1 = Number of Treatments Excluding the Control


V


2


3


4


5


6


8


10


5


3.03


3.29


3.48


3.62


3.73


3.90


4.03


10


2.57


2.76


2.89


2.99


3.07


3.19


3.29


15


2.44


2.61


2.73


2.82


2.89


3.00


3.08


20


2.38


2.54


2.65


2.73


2.80


2.90


2.98


30


2.32


2.47


2.58


2.66


2.72


2.82


2.89


60


2.27


2.41


2.51


2.58


2.64


2.73


2.80


2.21


2.35


2.44


2.51


2.57


2.65


2.72

Source: Dunnett, C. W. (1964). Biometrics, 20, 482-491.


and the difference in the true means is, with 95% confidence, within the interval:


-1.01 < y — yj < 1.01


We can say, with a high degree of confidence, that any observed difference larger than 1.01 Hg/L or smaller than -1.01 ^g/L is not likely to be zero. We conclude that laboratories 3 and 1 are higher than 4 and that laboratory 3 is also different from laboratory 5. We cannot say which laboratory is correct, or which one is best, without knowing the true concentration of the test specimens.

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