Subject: Re: MDS distances From: Sylvain Choisel <sc@xxxxxxxx> Date: Wed, 21 Jun 2006 11:47:46 +0200Have you tried with ALSCAL? http://forrest.psych.unc.edu/research/alscal.html If you want to avoid the trouble of formatting your data for ALSCAL, you can call it from Matlab with the following wrapper: http://acoustics.aau.dk/~sc/matlab/alscal.html Best regards, Sylvain > Dear Jim, > > I think the motivation behind your MDS question is a very interesting > one. To me it seems that there are two likely possibilities. > > -one is that "auditory dissimilarity" is truly quite high-dimensional, > so Matlab did not manage to project the data into a low dimensional > space because it simply cannot be done, and even reducing the > dimensionality just a little bit from 10 to 8 leads to appreciable > distoritions (i.e. "error"), > > -the other (which seems to be the possibility you are asking about) is > that Matlab's algorithm doesn't work properly. However, you could quite > easily "sanity check" Matlab's algorithm: Just draw ten points randomly > on (2-dimensional) graph paper, measure their pairwise distances, and > feed those into the Matlab MDS algorithm. If Matlab does its job right > it should give you a 2 D solution with errors no larger than the > measurement errors you would expect from holding your ruler up to the > points on your paper. In fact, it should be easily be possible to > "virtualize" and automate his sort of sanity check by letting random > numbers generate points in space of any dimensionality you choose and > work out relative pairwise distances using Pythagoras. Once you have > automated code that does this you could run hundreds of sanity checks > like that you should know pretty quickly how far you can trust Matlab's > algorithm. And I hope you will let us know the answer. I, for one, would > not be too surprised if it turned out that sounds could sound dissimilar > on at least 8 "different dimensions". > > Best wishes, > > Jan > > On 15/06/06, *beaucham* <beaucham@xxxxxxxx > <mailto:beaucham@xxxxxxxx>> wrote: > > We ran an MDS calculation (using MatLab) on a 10x10 distance > matrix based on dissimilarity judgements between all pairs of > 10 sounds, and obtained an 8-dimension solution, which gives > the coordinates of the sounds in 8-D space. The distances > between the positions of the sounds are supposed to match > the original distances. In fact, we get an rms error of 8% > and a max error of 30%. > > Is this typical? Is MatLab's program accurate? Is there a way > to improve on the MDS results? > > Jim Beauchamp > Univ. of Illinois Urbana-Champaign > jwbeauch@xxxxxxxx <mailto:jwbeauch@xxxxxxxx> > > > > > -- > Dr Jan Schnupp > University of Oxford > Dept. of Physiology, Anatomy and Genetics > Sherrington Building - Parks Road > Oxford OX1 3PT - UK > +44-1865-272513 > www.oxfordhearing.com <http://www.oxfordhearing.com>