From “The Top 25 US Public Libraries’ Collective Collection, as Represented in WorldCat” (OCLC), prepared for a confernce on the National Digital Public Library:
[a] large proportion of materials (nearly 90 percent) ... are held by five or fewer of the 25 institutions. In contrast, less than 1 percent of the publications are held by all twenty-five institutions, and only 4 percent are held by more than ten. These results suggest that the individual institutional collections that make up the collective collection are, at least at the publication level, characterized by a considerable degree of uniqueness vis-à-vis their peer collections. In other words, each institutional collection supplies a significant contribution to the scope and depth of the overall collective collection. In contrast, the core set of materials that are held by all, or at least most, of the institutions is comparatively small.