My colleagues (Lynne Connaway, Ed O'Neill and Chandra Prabha) recently did some work with colleagues in Vanderbilt to explore the characteristics of 'last copies', items which have only one holding in Worldcat. They have published a paper on the results:
Connaway, Lynn Silipigni, Edward T. O'Neill, and Chandra Prabha. 2006. "Last Copies: What's at Risk?" College and Research Libraries, 67,4 (July): 370-379. Pre-print available online at: http://www.oclc.org/research/publications/archive/2006/connaway-crl07.pdf (PDF:151K/24pp.)This work was encouraged by Paul Gherman, who has been advancing discussions on several fronts about the collective management of print collections. Indeed, as off-site storage and mass digitization initiatives proceed apace, consideration of the 'aggregate' collection grows in importance. In that context, the more we know about the characteristics of collections and how they compare becomes of more interest.