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LeMasters, Gary S.; Doyle, Douglas J. / Grade A dairy farm well water quality survey
[DNR-058] (1989)

Methods,   pp. 3-10 PDF (430.5 KB)


Page 7

Laboratory and the State Laboratory of Hygiene and there was close agreement
between the two laboratories. The WDATCP laboratory analyzed the milk for
the pesticide(s) detected in the water; none of these pesticides were detected in
any of the milk samples. Pursuant to Chapter Ag 161 of the Wisconsin
Administrative Code, a preliminary investigation was conducted at each farm
where an ES was exceeded to attempt to determine the source(s) of the
contamination. The sample results were explained in person to the farm
operator and information was collected concerning pesticide application history,
pesticide handling practices and well construction.
Statistical Analysis
Proportion Estimates
An unbiased estimator of the proportion of elements in a finite population
with a specified trait can be obtained from the following formula for a stratified
random sample (Scheaffer et al., 1979):
AA
pAt   =     [1/N] E N A
where
N     =     the number of elements in the target population.
N     =     the number of elements in the ih stratum.
=     the proportion of elements in the il stratum exhibiting the
specified trait.
The unbiased estimator of the variance for this proportion estimator is given by
(ibid.):
A2                  2AI
V(p.) =     [1/N2]   N,2  ,-n
NiN    ni - 
where
ni    =     the number of sampled elements in the i stratum.
A
qi =     1 -pi
The variance estimator can be used to derive confidence intervals as follows
(Ott, 1984):
A                  A    A
[ - t0.o5 (v(.),1/2) < P5, _< p + t.o5 (v(:p)"1/2]


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