Ullage, the word for the empty space at the top of a wine bottle, is Peter Suber's term for the gap between a library's actual holdings and its patrons' access needs. That's a difficult thing to measure, but I might have found a way to estimate it with reference not to patron needs but to all published journals, as follows.

• In 2003, Kathy Varjabedian at LANL compared the electronic holdings at 12 (large, well funded research) libraries with the ISI Journal Citation Report's top 100 most-cited journals for the previous year, producing an estimate for the ullage of those libraries of between 2% and a startling 54% (or 0% and 40%, if clinical titles were excluded).
• Also in 2003, Carol Tenopir estimated that there were around 44,000 scholarly journals in publication, just over 21,000 of them "refereed", which is the best proxy that Ulrich's Periodicals Directory allows for "peer-reviewed". Repeating Tenopir's search just now returned 26,677 active, refereed, academic/scholarly journals.
• Last year, I used a UCOSC dataset from 2004, a curated list of about 3000 titles, to estimate the average subscription price for a peer-reviewed scholarly journal (table 2 here) at $1238/title.

• Here are some more data from the Library Journal Periodicals Price Survey:
  
  
  
  Sorry about the jpg, I still can't make MT cope with tables. The spreadsheet is here. In case the image goes awry: the dataset covers more than 5,000 titles from 30 disciplines, and mean price/title is $723 in 2003 and $791 in 2004.
• The mean serials expenditure for an ARL member institution was around $5.5 million in 2003 and $5.8 million in 2004.

At $1200/journal, $5.8 million¹ would buy subscription access to about 4,800 titles, which is less than 23% of the number of active, refereed, academic/scholarly journals. At $700/journal, ARL members -- some of the largest and best funded libraries in America (indeed, in the world) -- are able to afford access to less than half of the scholarly literature.

This seems reasonably consistent with the earlier LANL estimate, given that Varjabedian looked only at the top 100 most-cited journals, which must surely be at the top of any research library's "must-have" list.

It's important to point out that what I'm estimating here is not ullage sensu Suber, but rather library holdings relative to all possible holdings. But I would argue that the access needs of all the scholars and other patrons served by ARL libraries is surely a decent proxy for "all possible journals", if not a significantly larger body of information! Put another way, here I am estimating the gap between current access levels and the information availability of a 100% Open Access world.




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¹This calculation assumes that 100% of the serials budget goes to scholarly journals. That's not true, but I've argued elsewhere that it's at least 90%.