[R] MDS with missing data?

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Hello

I will be applying MDS (actually Isomap) to make a
psychological
"concept map" of the similarities between N concepts.

I would like to scale to a large number of concepts,
however, the
resulting N*(N-1) pairwise similarities is prohibitive
for a user survey.
I'm thinking of giving people random subsets of the
pairwise
similarities.  

Does anyone have recommendations for this situation?

My current thoughts are to either

1) use nonmetric/gradient descent MDS which seems to
allow missing data, or

2) devise some scheme whereby the data that are ranked
in common
   by several people is used to derive a scaling
factor for each
   person's ratings.

Thanks for any advice,