-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 10-10-31 07:08 PM, Karen Kotschy wrote:
> Thanks, Philip, for your insightful questions and helping me to think > about the data more clearly. I was being stupid with the zeroes: yes, they > do result from aggregating the data, and they do represent cases where a > species did not occur in a particular sampling unit (so no cover or > abundance recorded). All records of abundance for a species have matching > records of cover. Since I am mainly interested in how strongly correlated > the 2 measures are, I think I can happily leave out the zeroes, since > I am only interested in abundance vs cover where these were recorded. You > have reminded me to think carefully about what the aggregation of my data > means for the analysis, though. Ben, my cover data is not in the form of > point counts so that is not an option. Also, I can't use raw counts for > abundance because of unequal sampling effort/area. OK, although if you have a measure of sampling effort you may be able to use offsets to adjust for it. > I have decided that correlation coefficients are probably fine for my > purposes. I have calculated Spearman and Kendall correlations, and used > Pearson correlations and model II regression on log-transformed data (as > you did, Etienne), as well as on ranked data. These all indicate a strong > positive correlation, and a linear relationship with transformed data, and > give a consistent picture. Sounds fine. > Carsten: did you imply that beta regression is necesarily model I > regression (no variance in predictor variable)?? I'd be interested to hear > anyone's thoughts on how much of a limitation this is for situations where > both y and x are random variables. Is it the same as for OLS regression, > where OLS is acceptable if the error variance in x is less than a third of > that in y? > I don't know exactly where that rule of thumb comes from ... 'model II beta regression' would correspond in my mind to fitting a bivariate probability distribution with Beta marginal distributions -- I know that this can be done using an object/approach called 'copulas' but this is a whole new can of worms which I have not opened myself ... Ben Bolker -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkzOL2gACgkQc5UpGjwzenO/QwCfXYAXUJLTQOivy0vIRkxVcrpf FHMAn23ZMcZCy8/s4bAYvqhG3XXUOO/+ =czEA -----END PGP SIGNATURE----- _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology