therefore, presence of 0 in some of your cells is not a math issue. Empty cells could easily compromise the strength of your conclusions, however.
[Side note that may be obvious to you: Catching 0 fish is not the same as an empty cell, where you never tried to catch anything. IN your discussion of precision, you will want to include some measure of how much time you spent catching fish at each site. If you caught 100 fish in 1 hour at a site, you were trying just as hard as the site where you caught 0 fish in 1 hour. Your precision estimate should include this fact in some way. If you use the number caught to determine the statistical precision, you may be fooling yourself.
When the count in a cell is below 5 (rule of thumb), the math may want you to adjust the value for non-normal issues. (you couldn't measure 3.5 fish at a site.) But if your catch time was nearly equal at all sites, you will have another angle to discuss precision.
2) You might want to compare counts across all sampling periods & species. This would be 26 x 4 columns (groups) in a one-way ANOVA test. It would probably say yes, there is a significant difference between some groups, but you wouldn't know if the difference was due to season, or species.
3) You might want to compare fish caught year annually by species. This would be 26 columns (groups) in your ANOVA, and it might tell you that some fish are more numerous than others. The tiny slice of data that you showed us suggests that much.
4) You might want to look at seasonal effects, within the 26 species. I _think_ those would be a nested design, or perhaps a two-Way ANOVA, depending on how you presuppose the model (way that fish fall out over the seasons). It might well be that you believe fish all peak in the same season (say spring), and reach a minimum in the same season (say winter). I _think_ this would be the 2-way ANOVA.
On the other hand, if you think that the peak season depends on the species, than I _think_ you want a nested design. Whichever, I urge you to discuss this with a statistician/analyst with experience in zoological work. They presumably will also have some ways to look at the precision and fishing time issue.
Good luck,
Jay
On Nov 14, 2007, at 9:23:43 AM, Rafael Rafael wrote:
Hello,I'm not an expert on ANOVA test and would like to receive helpto solve one problem. I will try to expose my doubts aboutthe test.I have a data set with specimens number (counts) of each 26 fish speciescaught by artisanal fishery on four season (summer, fall,....)My table is:Summer Fall...............................Winter........Replicate Sp1 Sp2 Sp3 Sp4.....Sp1 Sp2 Sp3 Sp4...........1 34 11 9 112 11 28 43 145..........2 27 17 15 136 6 15 21 89............3 33 21 12 109 0 19 33 115..........4.......5....... 0 0 06.......7...... 0 08......My doubt:What is the corret methods to test differences in the seasonal species composition ?I have some zero in the table. The species is absent in some replicates and in some seasons. Is there some problem with this zero in the table ?Any help to solve the problem to find the correct test for these data will be welcome.Thank you in advance for your attention to my request.All the best,RafaelSão Paulo - Brazil
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