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Ann,

The problem you identify is one that affects other network variables as
well.  For example, are two isolates structurally equivalent (they have
zero Euclidean distance) or not (they have an undefined alter profile
correlation because their columns are all zeros so have no variance)?
Burt distinguished constraint (the ability to do what one wants without
resistance) from power (the ability to do what one wants despite
resistance).  In that sense, your isolate is unconstrained because it has
no resistance.  On the other hand, its power may be undefined because
there's no resistance to see if it can overcome.  Your primary concern, I
think, is your last point:  whether, and what kind of, relations the
isolate can establish, since it's difficult to imagine being able to
accomplish anything without some ties.  As it stands, the isolate seems
unconstrained but unable to occupy a structural hole or other advantaged
position.  Since occupancy of an advantaged position is what people
usually care about when they measure constraint, that might explain why
the isolate's constraint is undefined in UCINET.

Mark S. Mizruchi               Phone: (734) 764-7444
Professor of Sociology and     FAX: (734) 763-6887
  Business Administration
University of Michigan
Ann Arbor, MI  48109-1382
homepage:  http://www-personal.umich.edu/~mizruchi/