On 29 May 2003 13:55:10 -0700, [EMAIL PROTECTED] (christoph) wrote: > sorry, maybe an all too trivial question. But we have power data from J > frequency spectra and to have the same range for the data of all our > subjects, we just transformed them into % values, pseudo-code: > > power[i,j]=power[i,j]/sum(power[i,1:J]) > > of course, now we have perfect collinearity in our x design-matrix, > since all power-values for each subject sum up to 1. > > How shall we solve this problem: just eliminate one column of x, or > introduce a restriction which says exactly that our power data sum up to > 1 for each subject?
Compositional data. A few days ago, someone posted news of a new edition (I think it was this book) by Aitchison. My stats-FAQ at http://www.pitt.edu/~wpilib/statfaq/compfaq.html includes about 8 other references. @Article{aitchison82, author = "J. Aitchison", title = "The statistical analysis of compositional data", journal = jrssb, year = 1982, volume = 44, number = 2, pages = "139-177", annote = "With discussion." I my own experience with power-spectra, I found it stabilizing to use the logit of the relative power. Of course, that got rid of the sum-to-1 problem. -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
