Roy Tennant (OCLC Research) has published a post on the Hanging Together blog entitled “Six Years of Tracking MARC Usage,” highlighting OCLC’s Ground Truthing the Use of MARC project and describing the reasons it was undertaken:
If we are to make a smooth transition to a world of linked data, we need to know what we have to work with. These reports are meant to expose exactly what we have to work with, in the aggregate, so we can make informed decisions.
It’s also possible to track the growth in the use of the newly-defined RDA fields using these reports, such as the 33Xs. For example, there were less than 200,000 records that had a 336 in 2013, but by five years later there were over 266 million. In contrast, use of the 258 Philatelic Issue Data field went from 8 appearances in 2013 to 19 in 2018. Not exactly meteoric.
dh+lib readers may be interested in exploring the MARC fields available on the Ground Truthing site; in his post, Tennant points out that MARC is still error-prone and deprecated subfields can achieve new but short lives. While OCLC has made available reports on many subfields, Tennant invites users to request reports for subfields of interest not present on the site.