Dear list ? My graduate student and I are using mvabund to analyze community data from an ecosystem-level manipulation experiment. The interaction term of interest (stream x period) is highly significant in the global multivariate test, but few taxa appear to be driving the difference, as shown by the univariate tests. I realize that this sort of result is to be expected at times, as the multivariate test is more powerful than each univariate test, but does anyone have general advice about follow-up analyses to shed more light on the nature of the significant result? Any suggestions would be greatly appreciated. Yours, Jon.
Question about mvabund results
2 messages · Jon Benstead, Martin Weiser
Dne 03. 10. 24 v 23:00 Jon Benstead napsal(a):
Dear list ? My graduate student and I are using mvabund to analyze community data from an ecosystem-level manipulation experiment. The interaction term of interest (stream x period) is highly significant in the global multivariate test, but few taxa appear to be driving the difference, as shown by the univariate tests. I realize that this sort of result is to be expected at times, as the multivariate test is more powerful than each univariate test, but does anyone have general advice about follow-up analyses to shed more light on the nature of the significant result? Any suggestions would be greatly appreciated. Yours, Jon. [[alternative HTML version deleted]]
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Dear Jon, as usual, the correct answer depends on what you would like to show :-) The multivariate test result itself basically says: "There is (quite probably) something going on." Nothing more, but that is enough for you to plot the differences and to be not accused of hallucinating the differences. You may stop there and let the readers interpret the (probably real) changes/differences themselves. (If you think that the result of the overall test is driven by the species with "positive" univariate tests - you may try to leave the species out or to replace their values with some random/permuted stuff, but be careful: see e.g. David Zeleny's, Stephane Dray and Pedro Peres-Neto's papers about that) Depending on the research question, you might follow up with plotting (showing, analyzing) various things: 1. Species with the highest relative change, or just showing the change in each species. 2. Change in some structural parameter, such as evenness 3. Change in the species abundance grouped by guilds/trophic levels/endangerment levels,... 4. ... Hypotheses resulting from the point 3 are testable in the multivariate framework: you just add them to the model. (Beware of statistical fishing - pun intended). HTH (and I hope someone corrects my naive views), Martin
Please expect long response time (3d+) - this is because of family reasons