[Warning, this is a sort of a brain dump/thought wander as I put together my thoughts about this topic.]

Someone at work pointed out this discussion of OPAC as tag clouds on The Gordian Knot. OPAC stands for Online Public Access Catalog, the database you would use in a library to search for titles and manage your transactions.

The exploration of different methods for displaying terms is interesting, but what I point out is that a tag cloud serves a different purpose than a vertically arranged list -- usually to display frequency of use of user-supplied keywords (freetags). That's why it's called a TAG cloud not a SUBJECT HEADING cloud, the difference being that tags are created and applied when the item being tagged is examined whereas the application of a subject heading involves the selection of a term from an authorized list that's already been developed and is thus usually more or less static (e.g. the worst case scenario, Library of Congress Subject Headings).

It's not particularly clear to me what systems would display from an OPAC if they're not dealing with user-supplied freetags. Perhaps we'd be dealing with either a) frequency of occurrences of the keywords or subject headings in the corpus (the OPAC) or b) number of items with a keyword or subject heading applied? If we're visualizing occurrences of keywords within the corpus that only tells us about how librarians catalog and about what the library collection contains rather than about what the patrons are using. I suppose, however, that you can correlate this frequency with popularity of loans to make it more meaningful. Tag clouds would be most interesting if they could show us how actual users perceive the collection, and unless I'm missing something here I don't think that's possible unless you allow freetagging.

The cloud display might not be particularly good for people who are interested in skimming lengthy lists of controlled vocabulary terms for known items, and that's the distinction I'd like to make, lest we start putting tag clouds all over information systems or using them in place of vertically-arranged lists. I think, however, when people read the entry on Gordian Knot or Shifted Librarian they might be get caught up in the demonstrations without really exploring the proofs of concept and how to make them useful. I'm curious to know if there are any OPACs out there that have some form of free tagging functionality built in. Clearly, those systems could do something with the cloud displays right away. Since the demonstrations in these threads deal not with free tagging, I wonder if cloud displays of terms are appropriate. I think you can make the case for cloud displays, however, if you execute on the right set of data. In our organization, for example, a programmer is doing some proof of concept for frequency of occurence of terms in our News source (Factiva), splitting up by facet (companies, industries, etc.), and that makes sense. We're visualizing the hot topics for the period being indexed and that seems to work. For example, showing the news tagged by company name, displaying the most frequent discussed companies can indicate which companies are being discussed most in the media, like a buzz metric. It would be really cool to apply this type of analysis against other sources as well, e.g. against weblog data using Moreover's service, for example.

But to get back to the discussion on the blogs, take a look at the example from Davey P's library blog. It shows a large list of subject headings from a database where a subject contains more than 10 items. Again, it tells you more about the collection and the cataloging than about usage. It is interesting as a visualization, but the thing about clouds is that they force you to work really hard if you are looking for known items, because vertical scanning for first letter occurrences is quicker than horizontal scanning. There's no reason why you couldn't do this as an alternative version for visualization of certain types of lists. But, the question really is, should you? I'm not offering any answers, I'm just playing devil's advocate.

Just to make the comparison of the two methods of display, take a look at this site's (urlgreyhot's) categories displayed as tag clouds and as vertical lists:

My blog tag cloud:
http://urlgreyhot.com/personal/tagadelic

Versus browsing the hierarchical lists of categories:
http://urlgreyhot.com/personal/sitemenu

The cloud is good for showing you which terms I applied most frequently, while the hierarchical list excels at being exhaustive and supporting skimming for known items. Granted the first example is simply a cloud representation of my controlled vocabulary rather than being a cloud of user-supplied freetags or keywords. But my point here is that each display serves different purposes and different types of information seeking tasks.

The appropriateness of the display should be determined by the nature of the information need or question the user demands of the system. I can't imagine that very many people coming to an OPAC would wonder, "Hmm, I wonder what subjects this library has the most books on?" Or alternatively, "What subject headings hold the most books in this library?, Let me browse the subject headings by number of books in the collection." Or maybe someone would want to know this. I don't know.

It could be very appropriate and meaningful to know, on the other hand what books are most popular (i.e. most borrowed) when browsing, especially when narrowing within a subject heading. I suppose that's more the realm of recommender systems, however, but still meaningful in this context. User-generated keywords would be a welcome addition to most OPACs and to other types of databases for that matter (see Headshift's BBC tagging demo, for example). But the problem with a freetagging cloud is that you'd have to have enough tags added to the system to make the display meaningful. I don't think there'd be a way to bootstrap that. You'd really have to put the tagging functionality in and wait for patrons to use it. I wonder how many people would tag an OPAC. I would guess very few, but it depends on the user group and how you incentivize freetagging.

These are all good things to explore, but before I advocate putting up tag clouds everywhere in our OPAC, I have to emphasize our focus on user needs/goals so we come up with the solutions that are most appropriate for meeting those needs rather than just throwing up more and more features into our information systems just because we can. The point is to design features to anticipate needs and information seeking behaviors. If a tag cloud anticipates a certain type of information seeking behavior, then it's appropriate. But you have to know and understand those behaviors and needs first. That's the part of the design process thats missing in these OPAC discussions.