Ok, this study was a follow up to a previous one, and here is what I would consider as the point to the study...this quote taken from the first study...
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However, there has never been a comparative experiment to determine the relative effects of sandbed depth, particle size and whether or not the presence of a void space beneath the sediments confers any advantage relative to the presence of the sediments themselves
I could not find a statement anywere in the presentation where they initially claim that the plenum offers no advantage over substrate alone as an actual hypothesis for the experiment, although that does seem to be a theme later when discussing the results and thier conclusions...
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Put simply, our experiment shows that the presence of a plenum has no measurable benefits over simply depositing the same sediments directly on the bottom of the aquarium (at least for nano-tanks over the time scales that we tested).
Ok...on the statistics...as I read it, they first compared their test results to the AutoAnalyzer results to determine mathmatical correlation. These test results (or dependant variables.... ie the salinity, pH, ammonia, nitrite, nitrate, oxygen, phosphate, alkalinity, and calcium) were then compared to the independant variable in each tank (ie deep, shallow, fine, coarse, plenum, no plenum) and were analyized with statistical models to determine whether the differences between the observed dependant variables were statistically significant enough to infer correlation with the independant variables.
Here's what they found...
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Analyses of variance for each water parameter revealed no significant differences among the final salinity, ammonia, nitrite, oxygen, or organic concentrations, nor were there any significant interactions among experimental treatments for any of these water parameters
So the independant variable did not correlate with the dependant varible in these cases.
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There were significant differences among treatments for the remaining water parameters
In the cases of pH, nitrate, phosphate, alkalinity, and calcium, there was a statistically significant difference between the dependant and independant varibles to correlate between them.
I find it worthy to note that livestock mortality was not listed as a dependant variable in the initial statistic summary, but it is inferred to later in the study, they did do some analysis of it...
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For the among treatment comparison, the overall analysis of variance was not significant (df = 7, F = 0.88, p > 0.5). However, there were nearly twice as many animal deaths overall in shallow as in deep sediment tanks (Fig. 9). On average 2.91 ± 0.46 animals had to be replaced in the shallow sediment treatments, whereas only 1.47 ± 0.46 animals had to be replaced in the deep sediment trials (df = 1, F = 5.23, p < 0.05). No other treatment or interaction term significantly affected the death rate in our experiment.
Basically, the mortality rate between tanks could not be correlated to the independant variable in a statistically significant manner.
On the whole, based on what can be inferred from the results of their experiment, these statements in the Overall Summary are the ones I have a problem with...(and lets face it, many people are going to breeze through the bulk of the article and focus on the overall summary, particularly newcommers to the hobby... )
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Overall death rates were roughly twice as high in aquaria with shallow sediments as in deep sediment treatments. The highest overall death rates were seen in aquaria with shallow coarse sediments over a plenum, and the lowest death rates occurred in aquaria with a sandbed composed of deep coarse sediments.
While this is their actual observation, it was NOT supported statistically that the reason the mortality rates in these tanks were due to substrate type. I think they should have made that point VERY clear in the summary! To read that summary, one might infer that the substrates correlated directly with mortality, which they simply didn't to a statistically significant degree.
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We did not test bare bottom tanks, but the data clearly suggest that the shallower the sediment, the higher the mortality rate
Whoa! Sure, they observed higher mortality in shallow systems, but as stated there was no statistical significance there, so one cannot infer with any degree of certainty that substrate depth and mortality correlate...which leads to this...
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and you can't get much shallower than a bare bottom tank!
Whoa! Now this is a big stretch! First, it wasn't in the test, so why even make a reference to it? Second, since no significat relationship was demonstrated on mortality, how can one possibly infer that?
MikeS