# 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 3.9 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 3.08 20 2.38 2.54 2.65 2.73 2.8 2.9 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 2.8 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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