RDA chapter three describes carriers. This is where you find all of the terms of measurement that appear in library data, things like:
box 16 × 30 × 20 cm
There is a controlled vocabulary in RDA for carriers. It has 54 entries that are in 8 categories:
projected image carriers
Note that one of the examples above, "map," is not included in the list of carriers. Nor is the most common extent used, "pages."* These are described in their own lists, "Extent of cartographic resource" and "Extent of text."** Why are these separate from other carriers? The answer is: Because they are not carriers, they are types of content. The carrier of a map is either a globe or a sheet, but map is not a carrier, it is a type of Expression, as is text.
It turns out that cataloging has been mixing content and carrier descriptions in the extent area for ... well, perhaps forever.
1 map on 4 sheets
1 atlas (xvii, 37 pages, 74 leaves of plates)
1 vocal score (x, 190 pages)
In addition, when describing books the carrier isn't mentioned at all, just the content:
xvii, 323 pagesunless there is no extent of the content to record, at which point the book is called a "volume:"
1 volume (unpaged)I have no doubt that there are clear rules that cover all of this, telling catalogers how to formulate these statements. Yet I am totally perplexed about how to turn this into a coherent data format. In FRBR, there is something called "extent of content" as an attribute of the Expression entity:
4.3.8 Extent of the Expression
The extent of an expression is a quantification of the intellectual content of the expression (e.g., number of words in a text, statements in a computer program, images in a comic strip, etc.). For works expressed as sound and/or motion the extent may be a measure of duration (e.g., playing time).
while "extent of carrier" is an attribute of the Manifestation entity:
4.4.10 Extent of the Carrier
The extent of the carrier is a quantification of the number of physical units making up the carrier (e.g., number of sheets, discs, reels, etc.).
RDA does not have "extent of content," in part (I am told) because it would have separated the instructions for formulating the extent of content and carrier between chapters 7 and 3, respectively, and thus made it difficult for catalogers to create this mixed statement. Of course, one possible response might be that we shouldn't be creating a mixed statement, but two separate statements that could be displayed together as desired. These statements should probably also be linked to the content or carrier vocabulary term that is now carried in MARC 336, 337, or 338.
I looked at ONIX to see how this might have been handled by another bibliographic schema, and it appears that ONIX has two different measures: extent, which is used for extent of the content, and measure, which measures the physical item.
We have to clear up inconsistencies of this nature if we hope to produce a rational format or framework for bibliographic data. Dragging along practices from the past will result in poor quality data that cannot interact well with data from any other sources.
I will add this to the analysis of MARC on the futurelib wiki.
* I can't find "box" anywhere in any list, but perhaps I am missing something.
** Extent of Text is even more complex than I thought. Here is the list of terms to be used:
approximatelyThese seem to not be extent of the text itself but the gathering of paper that something, mostly text possibly, is printed on. Volume is a carrier, as is leaf or page or case. However, approximately is totally out of place. Incomplete seems to be a statement about the content, although I suppose you could say that the carrier is incomplete when pages are missing. Note that sheet is here, but not in the list for cartographic resources, so it seems that in describing the carrier for a map one would use sheet from Extent of Text.
in various foliations
in various numberings
in various pagings
unnumbered sequence of pages