[R] question on isoMDS

[Helmut Kudrnovsky]

Hello everyone,

I have a question on isoMDS.

My data set (of vegetation) with 210 samples is in this way:

               Rotfoehrenau Lavendelweidenau Silberweidenau ....
067_Breg.7               0                2              0         ....
071_Dona.4              0                2              6          ....
...

I want to do an isoMDS-analysis with the dissimilarity index "bray/curtis" 
as discribed in the help-files of the package vegan:

mds.test <- isoMDS(test.bc, initMDS(test.bc), maxit=200, trace=FALSE, tol=1e-7)

Then I get following error message:

"Error in isoMDS(test.bc, initMDS(test.bc), maxit = 200, trace = FALSE,  :
         zero or negative distance between objects 19 and 20"

The objects 19 and 20 have the same variables in the same way and therefore 
they have a zero distance.

My question is: Is it possible to perform an isoMDS with a data set where 
some samples have the same variables in the same way?

with greetings from Tyrol
Helli Kudrnovsky

R: 1.8.1
OS: Win98