I see this figure frequently bandied about, and I think that it misses the point to some degree. Perhaps less than 20% of *everything* described in WorldCat or LC’s catalog have more than one manifestation, but for a public library, the percentage of new (and old) acquisitions that exist in more than one manifestation and/or more than one expression is significantly higher, quite possibly as high as the inverse percentage.
This is an excellent point, but I don’t know if I would go so far as to say the inverse percentage. I have also wondered how many of these are for obsolete editions of textbooks, or for materials that just probably won’t be used anymore, such as a microform of a public domain book when a copy can be found online for free. More precise research on this would be very useful.
But just as interesting is the case of a document in XML that uses various style sheets to generate all different kinds of formats: .doc, .pdf, .txt, whatever. All you need is a single source file in XML (this would be equivalent to the “expression”) and the different manifestations are generated. So, if you had a database of 10,000 documents and all were in XML, to make those same 10,000 documents available in an audio format, you could generate them automatically through a single XSLT file that transforms the source file. It could even be done on the fly. If you wanted to make them available in some new format, e.g. epub or mobi or some other version, it can be done by adding only one file. Do we want to catalog these sorts of items separately?
The first time this came up for me in an email discussion was back on the DC list, when we were discussing the current hot issue “1:1” and I asked how many records this item should be cataloged as: https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind9904&L=dc-general&D=0&P=64390 (the URL for the item has changed to http://www.ccel.org/ccel/kempis/imitation/imitation.html) There are several versions here and I don’t know if they are generated automatically or not, although they certainly could be. My original message mentions some formats that are no longer available (e.g. hypercard), so if it was decided to catalog the new formats of the 10,000 documents I mentioned above, those 10,000 items could just as quickly disappear. What is the best way to handle these sorts of issues that potentially create or save massive amounts of work?
But I want to emphasize that whether the public needs or wants to limit to specific editions/manifestation is irrelevant in my opinion. It is vital for the *librarians* to have this information because it is their job to manage the collection so it must be entered somewhere. How those records are displayed to the public and how it is engineered is another matter entirely and there can be great versatility today. For instance, the FRBR displays (yes, there are FRBR displays) where the records for differing editions can be merged into a single view for the public could be done now and does not have to be done by manually creating a different record structure. (See http://www.loc.gov/marc/marc-functional-analysis/tool.html)
Much more is possible too.