Dear list, i am not an ecologist by training, so please bear with me.
It is my understanding that Bray Curtis distances seem to be sensitive to different community sizes. Thus, they seem to deliver inadequate results when the different community sizes are the result of technical artifacts rather than biology (see e.g. Weiss et al, 2017 on microbiome data).
Therefore, i often see BC distances made on relative data (which seems to be equivalent to the Manhattan distance) or on data which has been subsampled to even sizes (e.g. rarefying). Sometimes i also see Bray Curtis distances calculated on Hellinger-transformed data,
which is the square root of relative data. This again makes sample sizes unequal (but only to a small degree), so i wondered if this is a valid approach, especially considering that the "natural" distance choice for Hellinger transformed data is Euclidean (to obtain, well, the Hellinger distance).
Another question is what different sizes (i.e. the sums) of Hellinger transformed communities represent? I tested some datasets, and couldnt find a correlation between original sample sizes and their hellinger transformed counterparts.
Any advice is very much welcome. Thank you. -- Dr. Tim Richter-Heitmann University of Bremen Microbial Ecophysiology Group (AG Friedrich) FB02 - Biologie/Chemie Leobener Straße (NW2 A2130) D-28359 Bremen Tel.: 0049(0)421 218-63062 Fax: 0049(0)421 218-63069 _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
