Algae Tile Data: 2004-2007, BPA-51; Preliminary Report, October 28, 2008. Page: 3 of 4
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increase over time.
Print Out 6 provides similar trend plots for the responses over sample sites. Lines, trends
and boxes are interpreted in the same manner as the previous printout. These plots show
chlorophyll production parameters to be variable over river distance with higher values occurring
mainly in the Downriver sites in 2004. Starting in 2005, the nutrient addition site, KR9.1, shows
higher levels of chlorophyll. This continues in 2006, but is mitigate in 2007 where chlorophyll
levels are elevated at all sites.
III) Determination of Sample Sizes
When estimating the mean, the formulation for calculating sample size is:
n = (z*s/d)2
where s, d and z are related to the variability, desired precision, and confidence levels,
respectively. As was stated previously, prior transformation of the data is necessary to meet the
distributional requirements for this estimation.
The desired precision was fixed at 10% of each response mean value. For the Downriver
Chlorophyll a example, 10% of the overall mean is approximately 2. The confidence level, z, is
obtained from a standard table of normal values, i.e.1.96 for 95% confidence. Standard
deviations, s, were obtained for each response and the resulting sample size estimates computed
(Print Out 7). Here, sample size estimates have be calculated for confidence levels of 90, 95 and
99%. All sample size estimates at the 95% level of confidence are within or close to the current
sampling protocol of 6 replications for 2004. For the remaining years, this is also true with the
exception of Chlorophyll a measurements where the sample size estimates are somewhat higher.
Most notable is the Upriver section in 2005 where estimates were all larger. This may be due to
more variable responses in that year. Overall, however, retaining the current level of sampling
intensity (6 replications) should ensure that the desired precision of 10% of the mean value can
be approximately achieved for all responses in the Upriver and Downriver categories with at least
It should be noted that for all the above calculations, the resulting sample size values are
preliminary. The calculations are based on limited data covering a single season. Furthermore,
the actual precision of mean estimates may vyy by sampling date. Therefore, these sample size
values should be used cautiously for setting policy regarding future sampling protocols.
IV) Additional Analysis
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Holderman, Charles. Algae Tile Data: 2004-2007, BPA-51; Preliminary Report, October 28, 2008., report, February 10, 2009; Portland, Oregon. (digital.library.unt.edu/ark:/67531/metadc933526/m1/3/: accessed January 20, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.