Participants in the cooperative NACO authority project are required to notify Library of Congress when the process of creating or modifying an authority record results in a need for bibliographic file maintenance (BFM) in LC's catalog, i.e., the correction of a heading on one or more LC bib records. Recent changes have reduced the number of cases which require this kind of reporting. [More in the extended entry].
The two major categories which no longer need to be reported are changes which effectively replace one version of a heading with another--e.g., a change which modifies a 100 heading; and changes which affect older records which LC does not distribute as MARC21 records.
The former category is discussed in detail in a FAQ posted to the NACO web site at:
http://www.loc.gov/catdir/pcc/naco/bfmguide.html
The site offers examples of the kinds of changes that need to be reported (e.g., two authors whose titles have been confused under a single name heading), and those which don't.
The second category involves records outside the scope of the BFM rule. Some are marked with the legend [from old catalog] in LC's browse indexes (at catalog.loc.gov). Other older records when viewed in LC's web catalog as MARC format records show in the 906 field the words "OCLC replacement." The answer to question 4 in the NACO FAQ on older records at:
http://www.loc.gov/catdir/pcc/naco/bfmfaq.html#4
instructs us that neither of these are candidates for BFM because they haven't been distributed by LC. You may find versions of older catalog records in OCLC flagged "D" (i.e., based on LC cataloging), but that's because some other library did copy cataloging from an LC card and correctly put DLC in the 040 $a; they're not actually distributed by LC as MARC21 records.
Of course, there is also an element of frustration in seeing a mistake and being asked not to report it; but that's the rule for now.
Authority control is a system librarians have devised to manage access to the records for their collections. When a number of items are held in a collection that have a common characteristic--same author, same topic, same series, versions of the same work--users of the collection may want to see the records for those items brought together. In online catalogs, authority control uses a separate file of records called authority records (distinct from bibliographic records, which represent items in the collection). The authority file defines uniform headings for each of these common characteristics--names, subjects, series, and entries for works. Also needed are rules for how to create and use these headings, and system functions to link the authority records to the bibliographic records and provide an integrated display of information from both. Authority records also enable the library to provide entries for variants of the uniform heading, and to indicate relationships between headings.
For example, if catalog users are searching for works by F. Scott Fitzgerald, they can enter a search for either "fitzgerald f scott" or "fitzgerald francis scott". Both searches will find the heading in the catalog where the author's works are listed: Fitzgerald, F. Scott (Francis Scott), 1896-1940. Authority control ensures that the only one form of the heading is used, and that it can still be searched in a number of ways. Similarly, a search on "fbi" will bring up a link to the heading for the body: United States. Federal Bureau of Investigation.
Authority records can also express relationships between headings. The authority for the FBI also points to a heading for the body's earlier name: United States. Dept. of Justice. Division of Investigation. The authority record for the topic Ostriches also points to broader and related topics: Birds, Ratites, and Cookery (Ostrich).
Often, authorized headings include more information than anyone would search for. This enables the system to display a clear and distinct heading in response to a partial or truncated search (e.g., "fitzgerald f s" leads to the full name heading above), and to distinguish between two similar entries (e.g., Wagner, Richard, 1813-1883 (for the composer) and Wagner, Richard, 1939-1972 (a dancer and choreographer).
Control freak is a blog devoted to library catalog authority control. It features answers to questions that I get about specific authority control issues, rules, and functions, and occasional comments on the state of the art. Additional questions and comments are always welcome.
Stephen