Statistics for Environmental Engineers

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Example 7.1


Twenty replicate samples from five stations were counted for plankton, with the results given in Table 7.2. The computed averages and variances are in Table 7.3. The computed means and variance


on the original data show that variance is not uniform; it is ten times larger at station 5 than at


2


station 1. Also, the variance increases as the average increases and sy seems to be proportional to y. This indicates that a square root transformation may be suitable. Because most of the counts


Station 2


Station 3


Station 4


Station 5



0    48    12    16 0    1    2    3    4



Station 1



0 10



0


10



ш


з


O-


CD



10



10



0


Plankton Count    / Count + 0.5


FIGURE 7.3 Original and transformed plankton counts.


TABLE 7.4


Eight Replicate Measurements on Bacteria at Three Sampling Stations


y = Bacteria/100 mL    x = log10(Bacteria/100 mL)


1


2


3


1


2


3


27


225


1020


1.431


2.352


3.009


11


99


136


1.041


1.996


2.134


48


41


317


1.681


1.613


2.501


36


60


161


1.556


1.778


2.207


120


190


130


2.079


2.279


2.114


85


240


601


1.929


2.380

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