Answer: Not many. And there are a lot of MARC tags that are rarely used. I recently analyzed the occurrence frequency of MARC tags in a December 2007 WorldCat snapshot prepared by our Office of Research colleagues. At that time WorldCat comprised 96,174,586 records and 1,210,107,485 holdings. The results don’t differ substantially from those Bill Moen presented from his extensive research on MARC designations at the 2006 RLG Members Forum: More, Better, Faster, Cheaper, even though WorldCat comprised just 56 million records at the time he had done his research.
Of the total 226 MARC tags represented in WorldCat.
- Only 27 tags occur in 10% or more of WorldCat records.
- 52 tags occur in 1% – 9% of WorldCat records.
- 147 tags occur in less than 1% of all WorldCat records.
Or 65% of all tags in WorldCat occur in less than 1% of all records. Bill Moen reported that of 167 unique fields identified in his research 66% occurred in less than 1% of all records. The record numbers change, but not the percentages, much.
The distribution of MARC tags in WorldCat looks like this.
I also looked at the tags weighted by the 1.2 billion WorldCat holdings attached to the records where these MARC fields appear, representing the number of items represented. The results are similar:
- 35 tags occur in 10% or more of WorldCat items.
- 48 tags occur in 1% – 9% of WorldCat items.
- 143 tags occur in less than 1% of WorldCat items.
Some tags are used more often by specific communities. For example, non-Latin script records are more likely to use uncontrolled subject terms (653 field, used in 18% of non-Latin script records) compared to the rest of WorldCat (4.27%). Vendor-supplied ordering data (in the OCLC-specific 938 field) occurs in more than half of all WorldCat items, although it is present in only 6% of all WorldCat records. Although form/genre terms in the 655 field occur in only 4.15% of all WorldCat records, it occurs in more than half of mixed material records (53.15%), 26.53% of visual material records, and 15.77% of integrated resource records.
Still, 40 tags occur in fewer than 1,000 records in a WorldCat database of over 96,000,000 records. Tags we can forget about since no one is using them anyway?