dc:identifier in wild ePubs

Here’s some trivia about the use of the required dc:identifier metadata element in ePubs that have been uploaded to Bookworm to date.

dc:identifier type
Percentage of total

None
0.5%

UUID
81%

ISBN
2.3%

URL
1%

Unknown format
15.2%

Although the ePub specification does not describe a particular identifier format, it is strongly recommended that ePub creators use one of ISBN, UUID or URL.
Note: only UUIDs and ISBNs that [...]